Diamond with inorganic coating such as silicon coatings was used to promote applications. Different kinds of silicon structures were found in the coatings. In this paper, first principle calculation was used to study the stability of cubic silicon and hexagonal silicon coatings deposited on diamond substrate. The simulation data suggest that the single layer hexagonal silicon coating form easily under low temperature due to the low lattice mismatching and thermodynamic temperature. However, the stronger bonding of Si-C bonds between substrate and coating direct the formation of cubic silicon coatings, as well as Si-Si bonds inner coatings, in agreement with experiments.