In this study, InGaN/GaN/Al(2)O(3)multi-quantum well (MQW) structure is investigated in 300-450 degrees C growth temperature range with steps of 50 degrees C. By the help of lattice constants at four different growth temperatures strain values are gained. Plot of strain in a- versus strain in c- is used to determine Poisson's ratio for GaN in the structure. Poisson's ratio is determined as 0.36. Reciprocal space mapping (RSM) technique of X-ray diffraction (XRD) method is used to gain peak positions of the sample instead ofw-theta scans because it gives more accurate results. Shifts in peak positions by variations of growth temperatures are shown. Lattice parameter equations are used with tau modification angles which minimise the errors caused from tilt of c- in hexagonal unit cell. This work is original because Poisson's ratio of GaN is determined using a natural method, variations in growth temperatures. It is also emphasised that RSM is a more sensitive method of XRD for determining structural properties of nitrite-based semiconductors.