Abstract: Based on the assumption of artificial electron mass amplification and prior work, a weighting scheme for rare species in DSMC simulation of chemical reactions involving ionization is proposed. The modification to the determination of inter-particle collision numbers introduced by the weighting factor is considered. The post collision particle states are determined according to the ratio of weighting factors. The chemical reactions involving ionization for 11 species air are treated as four classes respectively, with basic idea of reproducing the rare species in resultants and preserving/deleting common species in reactants according to the weighting factors. The impact of the usage of weighting scheme for chemical reactions involving ionization on electron density distributions and flow structures around space vehicles are analyzed by taking the reentry of RAM-C Ⅱ and Stardust as examples. The computations reveal that, the usage of proposed weighting scheme brings little changes to the macroscopic flow parameter distributions, while it significantly improves the smoothness of rare species contours such as electron for the case of weak ionization, and also improve their smoothness in low number density zone for stronger ionizations.