An investigation show that the temperature-induced band gap (E(g)) variation of PbSe nanocrystals is strongly size-dependent. The temperature coefficients (dE(g)/dT) evolve from negative to zero and then to positive values, with the increase of PbSe nanocrystal sizes. Such phenomena imply that PbSe nanocrystals may be the potential candidate as sensitive temperature markers. Additional analyses disclose that the molar extinction coefficients of PbSe nanocrystals remain unchanged in the investigated temperature range (25-120 degrees C).