Loco-regional thermal ablation of tumors is a conventional local therapy that uses high temperature to induce tumor cell apoptosis or coagulative necrosis. It has many potential benefits over surgery, including lower morbidity, increased preservation of surrounding normal tissues, reduced cost, and shorter hospitalization time.1,2 The loco-regional thermal ablation techniques that are clinically available are LITT (laser-induced thermal therapy), RFA (radiofrequency ablation), and HIFU (high-intensity focused ultrasound), which use a laser, radio frequency, and ultrasound, respectively, as the heat source. The challenge in using such techniques is to eliminate abnormal tissue without causing damage to the surrounding normal, healthy tissue. Thus, temperature monitoring and complex simulations that determine the intensity, direction, and shape of the thermal source have been conducted to localize the appropriate amount of heat at the target region.3–5 Despite such careful attention to the target, treatment of tumor tissues that are irregularly scattered or in complicated radial shapes often damages healthy tissues. That problem can be addressed using therapeutic techniques such as target-specific functional nanoparticles, also known as nanoprobes. When surface plasmon resonance occurs on the nanoparticle surfaces under laser irradiation, only the targeted tumor tissue containing the nanoparticles is heated, protecting the normal region from damage regardless of tumor shape or distribution.6 In this regard, plasmonic photothermal therapy (PTT) has been studied. PTT combines laser therapy and nanoparticle technology to enable tumor-specific heating. Nanoparticles with tumor-specific molecules, such as an antibody or aptamer, enable selective tumor treatment by PTT. Gold nanorods (GNRs) have been widely used in PTT studies. GNRs can support a higher-absorption cross-sectional area of near infrared waves per unit volume than other types of nanoparticles, and they exhibit a much narrower linewidth than spherical nanoparticles at similar resonance frequencies due to their reduced radiation damping effect.7 In addition, GNRs have been identified as optimal nanoparticles because they can be synthesized in bulk, have broadly tunable plasmon resonance, and allow easy surface modification. Various cancer treatment and imaging studies using GNRs have been conducted.8–11 Although PTT cancer treatment using GNRs has been quite effective, accurate temperature monitoring around the GNRs is required to minimize unintended effects on nearby normal cells. Many studies have been performed to monitor the temperature distribution in regions with and without nanoprobes using visible, mid-infrared, terahertz, and magnetic resonance (MR) imaging.12–16 Among those options, MR thermometers have been spotlighted as a non-invasive, non-depth-limited tool that does not require ionizing radiation, offers excellent anatomical resolution, and is useful in all scan directions.2 MR thermometers can measure and profile temperature using several methods, including proton resonance frequency, proton density, magnetization transfer, and diffusion coefficient.17–19 They are suitable for LITT and PTT applications because they do not interfere with the laser. Even with HIFU, MRI-compatible multi-element ultrasound transducers can be used to provide spatial control of the heating zone.2 RFA is not recommended with MR thermometry unless proper filtering is implemented because interference between the RF and MR systems is a serious concern.20 Although it is important to understand the diffusion temperature trends around the tumor, previous MR thermometry studies in PTT have mostly used spherical gold nanoparticles; no previous studies have used GNRs.21,22 Accurate temperature monitoring studies are needed because GNRs can absorb 3–5 times more light energy from plasmon resonance than spherical gold nanoparticles at the same gold mass.23 This study used MR thermometry to measure the photothermally induced temperature distribution in a GNR-implanted tumor phantom and a tumor phantom–implanted mouse model. The contours of each temperature distribution within specially selected ranges in the tumor phantom are displayed as binary temperature maps. Analysis of those binary temperature maps shows that MR thermometry can be used to optimize laser irradiation conditions. The temperature distribution and thermal diffusion of the GNR-implanted tumor phantoms were verified in vivo using MR thermometry in mice with implanted tumor phantoms. Those experiments confirmed the different tendencies of temperature increases and decreases depending on the presence or absence of GNRs and showed that it is possible to monitor differences in the degree of heat diffusion into surrounding tissue. These results demonstrate that MR thermometry has potential as a monitoring system to confirm the therapeutic effect of PTT and minimize the side effects to normal tissue.