Based on metal organic chemical vapor deposition (MOCVD), growth mechanism and stress modulation of van der Waals (vdW) heteroepitaxial GaN microwave material were studied with few-layer BN as an interlayer on 4-inch sapphire substrates. The influence of AlN nucleate process on growth mechanism of GaN buffer layer and its correlation with crystalline quality, stress, and electrical properties were discussed. A stress modulation scheme based on AlN/AlGaN composite nucleation process is proposed, achieving stress well in control for large-size vdW heteroepitaxy firstly. The as-grown GaN microwave material possesses a wafer bow of +20.4 μm, fullwidth at half maximum of GaN (002)/(102) peaks of 471.6/933.5 arcsec, root-mean-square roughness of 0.52 nm and electron mobility of 2 000 cm2/(V·s). Finally, large-size wafe-scale GaN microwave material was successfully separated from sapphire substrate by a mechanical lift-off process, providing convenience for transfering to high thermal conductivity substrates and creating conditions for fabricating high-power RF devices.