A diamond (111) substrate cleaned with an NH3/H2O2 mixture could form low-temperature direct bonding under atmospheric conditions. When the diamond surface was bonded with a plasma activated SiO2 surface at 200 °C, the bonding strength was sufficiently high so that cleavage within diamond occurred in a shear test. Moreover, the diamond and Si substrates treated with the NH3/H2O2 mixture could form atomic bonds with a 2.5-nm-thick oxide intermediate layer. This bonding method can be applied to electronic devices using diamond because heterogeneous integration can be achieved using a popular wafer cleaning process followed by low-temperature annealing.