Variations of fluorescence intensity ratio of green (generated from Er 3+ 2 H 11/2 and 4 S 3/2 levels) and red (generated from the sublevels of Er 3+ 4 F 9/2 level) upconversion emissions in Er 3+ /Yb 3+ /Li + :ZrO 2 nanocrystals have been studied as a function of temperature under 976nm laser diode excitation. In the temperature range of 323–673K, the maximum sensitivities of about 0.0134K −1 and 0.0104K −1 are obtained by using green and red emission, respectively. Er 3+ /Yb 3+ /Li + :ZrO 2 nanocrystals show potential application value in nanoscale thermal sensor.