Abstract: Homoepitaxial growth of single crystal diamond on a diamond substrate prepared by the high pressure high temperature method was carried out by microwave plasma chemical vapor deposition and the effects of methane concentration and substrate temperature on the quality of the as-grown diamond were investigated. The diamond was characterized by scanning electron microscopy and Raman spectroscopy. Results show that step growth and hillock growth are two main homoepitaxial growth modes, where a polycrystalline structure appears to form easily in the latter case. The surface roughness of the diamond is effectively improved by decreasing the methane concentration because it suppresses hillock growth. The homoepitaxial growth rate increases with increasing methane concentration, pressure and substrate temperature. However, the quality of the diamond is decreased by increasing the methane concentration and substrate temperature above 2% and 1 150 ℃, respectively. The content of amorphous carbon increases with growth layer thickness. A diamond with a high crystallinity and low defect concentration is obtained under optimal conditions.