The optimum surface coverage of diamond by hydrogen during chemical vapor depositon processes has been studied. It is assumed that the reduced chemical reaction of the singlet state of the surface dangling-bonds is different from that of the dangling-bonds-clusters in which the dangling-bonds may combine each other at the surface and form non-diamond structrures. The singlet states of the surface dangling bonds may be the main templets contributed to the diamond growth. It has been observed that the optimum surface coverage is 0.86 monolayer (ML)for {111} oriented surface , and 0.80 ML for {100} oriented surface for diamond growth, and it may be the maximum growth rate at a definite concentration of hydrocarbon fragments at the surface.