Synthesizing diamond single crystal by diamond seed particles which were electroplated with nickel film as catalyst under high pressure and high temperature(HPHT)was described.The microstructure of nickel film after synthesis and morphology of grown diamond were investigated by means of X-ray diffraction(XRD)and scanning electron microscopy(SEM),respectively.The phase structure in nickel film were graphite,NiC,Ni, and diamond structure hadn’t been found.A lot of recrystallized graphite pits appear in interface between the inner surface of nickel film and the surface diamond.It is shown that the new-grown diamond was developed epitaxially on the crystal planes of seeds.Also,the new-grown diamond grew by two-dimensional nucleation and by a layer growth mechanism.The growth process of crystal was microaggregate→step→expansion→new crystal layers,and the flat growth interface transformed into a cellular interface at the same time. Synthesizing diamond single crystal by diamond seed particles which were electroplated with nickel film as catalyst under high pressure and high temperature (HPHT) was described. Microstructure of nickel film after synthesis and morphology of grown diamond were investigated by means of X-ray diffraction ( XRD) and scanning electron microscopy (SEM), respectively. The phase structure in nickel film were graphite, NiC, Ni, and diamond structure had not been found. A lot of recrystallized graphite pits appear in interface between the inner surface of nickel film and the surface diamond. It is shown that the new-grown diamond was developed epitaxially on the crystal planes of seeds. Also, the new-grown diamond grew by two-dimensional nucleation and by a layer growth mechanism. microaggregate → step → expansion → new crystal layers, and the flat growth interface transformed into a cellular interface at the same time.