Molecular mechanics simulation was employed to study the adsorption of copper phthalocyanine(CuPc) on HOPG and alkane adlayer. It is shown that CuPc and C34H70 have similar adsorption energies on graphite surface. Attributed to the strong molecule-molecule interaction and the large lateral diffusion barrier, the two-dimensional monolayer of C34H70 is preferentially formed, which is consistent with STM observations. Due to the lateral corrugation barrier of CuPc on alkane and the roughness of alkane monolayer, the diffusion mobility of CuPc on alkane layer was decreased and the stability of two-dimensional monolayers of CuPc was improved.