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SUMMARYLow-pressure or ultra-high vacuum chemical vapour deposition often involves important trace species in both gas-phase and surface reactions. The conservative weighting scheme (J. Thermophys. Heat Transfer 1996; 10(4):579) has been used to deal with the trace species often involved in some non-reactive physical processes, which is otherwise considered computationally impossible using the conventional DSMC method. This conservative weighting scheme (CWS) improves greatly the statistical uncertainties by decreasing the weighting factors of trace-species particles and ensures the conservation of both momentum and energy between two colliding particles with large di erence of weighting factors. This CWS is further extended to treat reactive processes for gas-phase and surface reactions with trace species, which is called extended conservative weighting scheme (ECWS). A single-cell equilibrium simulation is performed for verifying both the CWS and ECWS in treating trace species. The results of using CWS show that it is most e cient and accurate for weight ratio (trace to non-trace) equal to or less than 0.01 for ows with two and three species. The results of a single-cell simulation using ECWS for gas-phase reaction and surface reactions show that only ECWS can produce acceptable results with reasonable computational time.