GaN substrates were directly bonded with Si substrates by wet treatments using H2SO4/H2O2 and NH3/H2O2 mixtures. Under the optimized condition, the tensile strength reached 7.36 MPa, and a part of the Si substrate was fractured within the bulk instead of the bonding interface. There is an amorphous intermediate layer with a thickness of 1.7 nm, which mainly consists of Si oxides, at the bonding interface. It is remarkable that wafer-scale GaN/Si integration was successfully achieved by using common cleaning methods. It is believed that the proposed direct bonding technique would contribute to future heterogeneous integration because the GaN and Si substrates can be bonded through the atomically thin intermediate layer without vacuum processes.