Abstract: The dimensions of the rigid groups in the VGO model compounds and the dimensions of VGO molecules are calculated by using molecular simulation techniques. Dynamics methods were applied to investigate the diffusion process of VGO compounds at 800 K in FAU zeolite. The diffusibility difference of different VGO model compounds was studied by calculated penetration distance. The results indicated that the carbon chain length of VGO molecule and the dimensions of rigid group are the main factors that influence the diffusion ability of VGO in FAU zeolite. The increase in chain length leads to a lower diffusion ability due to the crimp of VGO molecules. The increase in ring number in naphthene and aromatics or the branched chain in the molecule increases the minimum crosssection size of the molecular rigid group of the hydrocarbon molecules， thus reducing the diffusion ability of the hydrocarbon molecules. When the hydrocarbon mixed with polycyclic aromatics as a model， the diffusion ability of the easy diffused molecules is lowered due to pore block by polycyclic aromatics.

Key words: diffusion coefficient, penetration distance, molecular demension