The present invention is a method for producing a non-polar a-plane GaN thin film on an r-plane sapphire substrate using metal organic chemical vapour deposition (MOCVD) technique. Particularly, the method provides suitable growth conditions for GaN layer during MOCVD by controlling V/III ratio, reaction temperature and flow rate of reactants, but not limited thereto. It was found that such method could provide non-polar a-plane GaN thin film on r-plane sapphire substrate with reduced lattice mismatch between each layer.