In this work, the wafer bowing during growth can be in-situ measured by a reflectivity mapping method in the 3×2 Thomas Swan close coupled showerhead metal organic chemical vapor deposition(MOCVD) system. The reflectivity mapping method is usually used to measure the film thickness and growth rate. The wafer bowing caused by stresses(tensile and compressive) during the epitaxial growth leads to a temperature variation at different positions on the wafer, and the lower growth temperature leads to a faster growth rate and vice versa. Therefore, the wafer bowing can be measured by analyzing the discrepancy of growth rates at different positions on the wafer. Furthermore, the wafer bowings were confirmed by the ex-situ wafer bowing measurement. High-resistivity and low-resistivity Si substrates were used for epitaxial growth. In comparison with low-resistivity Si substrate, Ga N grown on high-resistivity substrate shows a larger wafer bowing caused by the highly compressive stress introduced by compositionally graded Al Ga N buffer layer. This transition of wafer bowing can be clearly in-situ measured by using the reflectivity mapping method. In this work, the wafer bowing during growth can be in-situ measured by a reflectivity mapping method in the 3 × 2 Thomas Swan close coupled showerhead metal organic chemical vapor deposition (MOCVD) system. The reflectivity mapping method is usually used to measure the film thickness and growth rate. The wafer bowing caused by stress (tensile and compressive) by the epitaxial growth leads to a temperature variation in different positions. the wafer bowing can be measured by analyzing the discrepancy of growth rates in different positions on the wafer. . In contrast with low-resistivity Si substrate, Ga N grown on high-resistivity substrate shows a larger wafer bowing caused by the highly compressive stress This transition of wafer bowing can be clearly in-situ measured by using the reflectivity mapping method.