Yb3+-Er3+ ion co-doped NaLnTiO4 (Ln = Y, Gd) up-conversion (UC) phosphors were successfully synthesized by a sol-gel method. The phase purity and the structure of the samples were characterized by powder X-ray diffraction (XRD), and the optimal compositions were also determined according to their UC emission intensities. The samples emit orange light and their UC spectra were recorded with excitation by a laser diode with a 980 nm wavelength. The UC luminescence intensity could be enhanced greatly after introducing sensitizer Yb3+ ions, and the energy transfer (ET) from Yb3+ to Er3+ plays a vital role. The UC mechanism and processes responsible for the emissions were investigated and found to involve two-photon absorption. The lifetime of green emission in Er3+ singly doped and Yb3+-Er3+ co-doped samples were measured to prove the existence of ET. The temperature dependence of the fluorescence intensity ratios (FIR) for the two green UC emission bands peaked at 530 and 550 nm was studied in the range of 300-510 K under excitation by a 980 diode laser with about 4 W cm-2 power density, and the maximum sensitivity was approximately 0.0045 K-1 at 510 K for NaYTiO4 and 480 K for NaGdTiO4. This indicates that Yb3+-Er3+ ion co-doped NaLnTiO4 (Ln = Y, Gd) phosphors are potential candidates for optical temperature sensors.