Abstract: Temperature effect caused by Doppler broadening of neutron cross section plays a significant part in neutronics calculation of nuclear reactor. Based on free gas model and through sampling the velocity of target nucleus randomly, On-the-Fly Doppler broadening can be carried out in Monte Carlo calculation using zero-Kelvin cross section, which eliminates the need of Doppler processing codes. The method and examples were shown in this paper. The capability of this method to deal with temperature-reactivity effect was proved, and the applicability and further development were also discussed.