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Serious wafer curvature and residual stress are formed during the growth of an epi-GaN layer on Sapphire substrates due to the different thermal expansion coefficients in these two materials. By using theoretical analysis and a simulation model 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 reduction. In addition a new 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 to ~ 37 mm from the original ~ 45 mm in 2 inch wafers with an optimized surface structure design.<br />