Microwave plasma chemical vapor deposition (MPCVD) was used to grow single-crystal diamonds on two types of single-crystal diamond seed substrates prepared by high-pressure, high-temperature (HPHT) and chemical vapor deposition (CVD) methods. The quality of diamonds grown on the different seed substrates was compared. Fluorescence characteristics showed that the sectors of the HPHT seed substrates were obviously partitioned. Raman and absorption spectra showed that the CVD seed substrate produced higher-quality crystals with fewer nitrogen impurities. X-ray topography showed that the HPHT seed substrate had obvious growth sector boundaries, inclusions, dislocations, and stacking faults. The polarization characteristics of HPHT seed substrate were obvious, and the stress distribution was not uniform. When etching HPHT and CVD seed substrates using the same parameters, the etching morphology and extent of different growth sectors of the two substrates differed. Although extended defects were inevitably formed at the interface and propagated in the CVD layer, the dislocation density of a 1 mm-thick CVD layer grown on a CVD seed substrate was only half that of a 1 mm-thick CVD layer grown on an HPHT seed substrate. Therefore, the use of CVD seed substrate enabled the growth of a relatively higher-quality CVD single-crystal diamond.