We report experimental results on the temperature dependence of band gap in size-controlled CdSe nanocrystals. Such temperature-dependent property is also size-dependent. Within a certain temperature range, the band gap absorption peak position (lambda in nm) of CdSe nanocrystals with a given particle size (D) can be expressed as a simple linear function of temperature (T in K): lambda = 0.1 T + C (D, 298 K). This function is practically applicable to any particle size and temperature. A comparison reveals that the band gap values calculated using this function are in agreement with the experimental ones reported previously. (c) 2007 Elsevier B.V. All rights reserved.