Sulfur and oxygen are known to improve the crystal quality of the chemical vapor deposition (CVD) diamond. In the CVD process, the sulfur is incorporated into the diamond crystal, while the oxygen is not incorporated. In the present study, first-principle calculations have been performed to investigate the effect of sulfur and oxygen on the growth mechanisms of CVD diamond (100) surfaces. The S and O atoms in the vapor are spontaneously inserted into the dimer bond on the diamond (100). The S and O insertions induce a compressive stress along the dimer bond and enhance the CH2 bridging across the trough. In the case of oxygen, the on-top C=O structure is spontaneously formed and it is considered to be desorbed from the surface during the CVD process. The S atom is considered to be incorporated into the diamond (100) lattice via the formation of 3-and 4-coordinated S on the surface.