Yb3+/Tm3+ co-doped LiNbO3 crystals were proposed to explore the temperature sensing characteristics of non-thermally coupled levels from Tm3+ ions. The prepared polycrystal is opaque and single crystal is transparent. Under 980 nm excitation, upconversion spectra were investigated at different temperatures changed from 323 K to 773 K. It is found the deep red (3F2,3 → 3H6) and blue (1G4 → 3H6) or red (1G4 → 3F4) emissions of Tm3+ ions exhibit opposite temperature dependent behaviors, so their temperature sensing characteristics are studied. The results indicate the temperature sensitivity is mainly affected by crystallinity of host material and concerned energy levels of FIR (fluorescence intensity ratio). As a consequence, the maximum relative sensitivity of about 0.7% K-1 is achieved based on the non-thermally coupled levels of Tm3+ ion in LiNbO3 crystals. The polycrystal reveals more excellent temperature sensing properties in low temperature, and single crystal is more suitable for high temperature application. Moreover, it is discovered the stability of FIR with different powers greatly depends on the concerned energy levels. This work may contribute a feasible and expansible way to further survey the sensitivity of optical temperature sensor and promote its applications.