We have investigated the surface morphology of GaN layer grown on the nitrided (0001) sapphire substrates by two-step MOCVD method using AlN buffer layers. The surface morphology of GaN grown layers was changed by both the nitridation process of sapphire substrates and ALN buffer layer thickness. The mirror surface was obtained for the growth on non-nitrided substrates using a relatively thin AlN buffer layer. On the other hand, the surface morphology of GaN layers grown on nitrided substrates changed from large hexagonally faceted surface to mirror surface with an increase of the A1N buffer layer thickness. From atomic force microscopy( AFM) measurement, it was found that the surface morphology of GaN layers was strongly related to that of annealed AIN buffer layers. From the difference of both thermal stability of GaN layers and low-temperature PL spectrum, the GaN layer having a different surface morphology is found to have a different polarity. That is, the mirror surface means (0001)Ga plane growth and the hexagonally faceted surface means (0001) N plane growth.