Abstract  Quantum chemistry method was used to calculate the geometry structure of heavy oil character molecule. The method to calculate the minimum cross-sectional diameter of heavy oil character molecule and to show the molecular dimension was brought forward, which was used to predict the minimum cross-sectional diameter of some typical heavy oil character molecules. It is shown that the theory calculated result was consistent with experimental date. The minimum cross-sectional diameters of Daqing first cut (DQ-fcut) , middle cut (DQ-mcut) and end cut (DQ-endcut) were 1.8703, 3.0080 and 3.4929nm, respectively, meaning that these kinds of heavy oil molecules are too big to enter the sieve pore of zeolites, so they could be only cracked on the surface of zeolite. These research results can offer some useful theory basis to the selection and synthesis of zeolites.