Because of the different thermal expansion coefficients in these two materials, serious curvature and residual stress are existed during the growth of an epi-GaN layer on Sapphire. By using theoretical calculation and a simulation model analysis using the finite element method to describe the realistic shape of wafer curvature on epi-GaN wafers, we examine the influence which different thickness and thermal expansion coefficients in the top epi-GaN layer have on wafer curvature released. In addition a new automatic process to reduce wafer curvature and to relax residual stress is proposed. With an additional laser treatment on a sample surface after the growth of the top epi-GaN layer on a Sapphire substrate has taken place, the wafer curvature can be reduced from the original 45 m to 37 m in 2 inch wafer with an optimized surface structure design.