The inclusion of cubic phase in MOVPE-grown hexagonal GaN on GaAs substrate and its dependence with the growth temperature are investigated by high-resolution X-ray diffraction (HR-XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and cathodoluminescence (CL). It is observed that the GaN layers surface exhibits 3D-grains structure. The density and shape of these grains are largely dependent on the growth temperature. HR-XRD study reveals the presence of cubic GaN clusters in the hexagonal GaN layer. Using CL we show that the cubic inclusions are not localized at the substrate/epilayer interface but propagate throughout the film.