We report on the surface reconstructions on the basal planes of wurtzite GaN. Depending on the surface polarity, two distinct classes of reconstructions form. The Ga-polar surface displays the Ga fluid-1 x 1, 10 x 10, 5 root3 x 2 root 13, 5 x 5, 4 x 4, and 2 x 2, and the (0 0 0 (1) over bar) N-polar surface the 6 x 8, 6 x 6, root7 x root7 and 2 x 3 with decreasing surface Ga coverage, We will show that their structure is consistent with a simple Ga-adatom-based scheme. Crystalline structure and optical properties of the GaN epilayers are examined by high resolution X-ray diffraction and photoluminescence, and the results reveal a strong relation between the threading-dislocation density and the intensities of both near-band transition at 3.427 eV and yellow luminescence at 2.16 eV.