Vernier and Fano resonances are promising approaches for enhancing the sensitivity of an alloptical sensor. A theoretical analysis was performed to integrate a Fano-like resonance shape with a Vernier resonance by considering the presence of partial reflective end facets at a double microring resonator waveguide. The system was developed based on scattering matrix and optical transfer function. The double and all-pass racetrack microring configurations were compared with and without the end facet at the waveguide to analyze the dynamic change of the output resonance spectrum. The spectrum were analyzed based on the free spectral range and resonance pattern. The resonator systems were applied to refractive index-based sensing protocol, which was operated by a shift of resonance wavelength with a change of refractive index. The sensitivity was optimized by varying the configuration parameters such as the radius of the ring, the distance between end facet, and the coupling coefficients. Integrating Vernier spectrum with Fano resonance improved the sensitivity for all-pass racetrack configuration by 5.16% and the sensitivity for double racetrack configuration by 6.31%. The recorded limit of detection (LOD) of double racetrack was 3.30 × 10 -5 .