Abstract: The diffusion of ethanethiol in MFI and MOR was investigated by Molecular Dynamics (MD). The results show that the diffusion coefficient decreases with an increase in the quantity of ethanethiol adsorbed. The zeolites of flexible and inflexible structures are different in diffusion coefficient. For MFI, the diffusion coefficients in flexible and inflexible structures are 3×10－10m2/s and 4×10－9m2/s, respectively; this order is reversed for MOR, the diffusion coefficients in flexible and inflexible structures are 2×10－7m2/s and 3×10－8m2/s, respectively. The diffusivity of ethanethiol in flexible MOR is higher than that in flexible MFI, because the 12 member ring in MOR is larger than the 10 member ring in MFI. For MFI, the adsorptive sites in both intersection channel and straight channels are similar in the energy of interaction; for MOR, however, some differences occur between 12 member ring and 4 member ring.