Strain evolution in GaN layers grown on flat (FSS) and cone shaped patterned sapphire substrates (CP55) is studied by in-situ reflectance and curvature measurements. Intrinsic growth strain and dislocation density are investigated in relation to different steps of the growth procedure. In spite of the typical tensile strain of GaN on sapphire, a clear compressive stress during growth is found when the lateral coalescence time is delayed. From the curvature measurement, the average compressive stress during GaN layer growth on CP55 before full coalescence is determined to be below 0.21 GPa. Subsequently, tensile stress of 0.25 GPa builds up as soon as the surface is closed. Using optimizing growth procedures with controlled stress profile enables nearly zero wafer bow in critical steps of the growth process. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.