As an optical temperature sensor, rare earth ions (Tm3+/Yb3+)co-doped (beta-NaYF4 upconversion nanoparticles (UCNPs) were synthesized via a thermal decomposition method. Fluorescence intensity ratio (FIR) between the 700 nm (F-3(2,3)-> H-3(6)) and 800 nm (H-3(4)-> H-3(6)) corresponding to the emission spectra of Tm3+ was studied in the temperature range of 300-498 K under 980 nm excitation as a function of optical thermometry. It was found that the temperature sensing sensitivity of Tm3+/Yb3+ co-doped beta-NaYF4 UCNPs was higher than that of other rare-earth doped materials which were reported previously by using FIR technique in terms of sensitivity. beta NaYF4:Tm3+/Yb3+ UCNPs were promising to be a temperature sensor with the high sensitivity and resolution, especially with the superior property of well separated emission bands (700 nm and 800 nm).