This study addresses a mechanism by which a diamond surface cleaned with a H2SO4/H2O2 mixture forms a direct bonding with an oxygen-plasma-activated SiO2 surface. Surface characterizations revealed that the cleaning with the H2SO4/H2O2 mixture at 75 °C effectively OH-terminated the diamond (111) surface without a significant increase in the surface roughness. The OH-terminated diamond and SiO2/Si substrates formed a C–O–Si bond by thermal dehydration reaction at 200 °C without significant loss in diamond crystallinity. It is expected that the bonding technique using H2SO4/H2O2 can contribute to the integration between diamond and semiconductor substrates because the high-crystallinity diamond substrate can be directly bonded through the commonly used cleaning processes.