A comparative study on mechanisms of radical initiated self-directed growth of styrene molecules on the H-terminated Si(111) and Si(100) has been carried out by using quantum chemical and molecular mechanics methods. Several possible H-abstraction pathways through formations of transition states containing five-, six-, and even eight-membered ring structures are investigated with the aid of surface Cluster models and density functional theory calculations. It has been demonstrated by employing periodic surface models and molecular mechanics simulations that the surface pattern and intermolecular interactions between phenyl groups play important roles in the self-directed growth processes The formation of cluster-shaped aggregation of styrene molecules on H-Si(111) results from the undirectional chain reactions, due to the isotropic hexagonal arrangement Of Surface sites. On the contrary, the anisotropic style of H-Si(100) induces a strong directional preference for H-abstractions, following an order of the inter Si-Si dimer > the intra Si-Si dimer >> the inter Si-Si dimer row. The one-dimensionally ordered structures of single and double lines along the Si-Si dimer row are thus formed on H-Si(100) The self-directed growths of styrene molecules on both H-Si(111) and H-Si(100) are revealed to be stage-dependent.