ThepurecubicGaN(c-GaN)hasbeengrownon(001)GaAssubstratesbyECR-PAMOCVDtechniqueatlowtemperatureusingTMGaandhighpureN2asGaandNsources,respectively.TheeffectsofsubstratepretreatmentconditionsonqualityofcubicGaNepilayerareinvestigatedbythemeasurementsofTEMandXRD.Itisfoundthathydrogenplasmacleaning,nitridationandbufferlayergrowthareveryimportantforqualityofcubicGaNepilayer.