Abstract: The free gas model is adopted to consider the thermal scattering effect of the elastic collision between neutron and target. The conventional model assumes that the elastic scattering cross sections are constant at 0 K, which neglects the influence of resonance effect. In order to consider the resonance elastic scattering in the free gas model, the Doppler broadening rejection correction (DBRC) method was applied to correct the free gas model of MCNP. The Mosteller’s Doppler defect benchmark for LWR pin cell was analyzed. The numerical results show that neglect of resonance elastic scattering effect contributes to overestimation of the infinite multiplicative factor to the extent of 40-100 pcm and 140-200 pcm for hot zero power and hot full power cases, respectively. The fuel temperature coefficients are also overestimated 7%-15%. The computational time of the newly developed sampling technique was studied and the influence of the resonance elastic scattering effect on the emergent energy distribution was analyzed.