A series of Bi3+ single-doped and Bi3+/Er3+ co-doped YNbO4 phosphors with various concentrations of Bi3+ ions were prepared by a conventional high temperature solid-state reaction method. The results of XRD and Rietveld refinement confirmed that monoclinic phase YNbO4 samples were achieved. The down-/up-conversion luminescence of Er3+ ions were investigated under the excitation of ultraviolet light (327 nm) and near infrared light (980 nm). Under 327 nm excitation, broad visible emission band from Bi3+ ions and characteristic green emission peaks from Er3+ ions were simultaneously observed, while only strong green emissions from Er3+ ions were detected upon excitation of 980 nm. Remarkable emission enhancement was observed in down-/up-conversion luminescence processes by introducing Bi3+ ions into Er3+-doped YNbO4 phosphors. By analyzing the laser working current dependent up-conversion luminescence spectra, two-photon processes were confirmed to be responsible for both the green and the red up-conversion emissions of Er3+ ion. The temperature sensing property of Er3+ was studied by using the temperature dependent up-conversion luminescence spectra and it was found that the temperature sensitivity was sensitive to the doping concentration of Bi3+ ions. By comparing the experimental values of the radiative transition rate ratio of the two green emission levels of Er3+ ions and the theoretical values calculated by Judd-Ofelt (J-O) theory, it was concluded that energy level splitting had significant influences on the temperature sensing property of Er3+ ions.