Diamond-based semiconductor with high electrical conductivity is a key point in diamond device development. In this paper, a thin single crystal diamond layer of high quality was epitaxially grown on a commercial tool-grade diamond seed by incorporating active O atoms from the typical growth environment. Subsequently, the H-termination density was enhanced on the diamond surface by exposure to the pure hydrogen plasma, and the surface conductivity of H-terminated diamond was analyzed in detail. The thin epitaxial layers on the high-pressure high-temperature diamond seeds show lower resistance than the ones on the chemical vapor deposition diamond seeds, which could be comparable with the lowest values reported. After the thin diamond layers were grown with and without addition of O 2 , the carrier mobility in the conductive channel increased to almost 80 cm 2  V −1  s −1 under O 2 contained condition, much higher than those without O 2 incorporation. The ionization scattering is dominant to the carrier mobility compared with the surface scattering. The higher carrier mobility is attributed to the lower impurity density in the epitaxial layer, which is because the active O atoms could purify the epitaxial layer by removing or reducing Si- and N-related impurities.