The effects of the trimethylgallium flow (14-55 mu mol/min) during the deposition of the GaN nucleation layer on the structure and electronic properties of GaN epilayers were examined. X-ray and mobility studies indicate that GaN epilayers, grown using non-optimal trimethylgallium (TMG) flow, result in wide FWHM peak and low electron mobility. On the contrary, an optimal TMG flow during the nucleation layer growth leads to films with superior structural and electronic properties. Atomic force microscopy (AFM) was used to systematically investigate the morphological evolution of as-grown nucleation layers, and the nucleation layers were heated to 1000 degrees C under different TMG flows.