Effects of the nitridation of the r-plane sapphire were investigated on the growth of a-plane GaN. Surface morphology and crystal quality were very sensitive to the nitridated time. A high quality a-plane GaN with a pit free-surface was obtained with nitridation at 1100 °C for 5 min, compared with under- or overnitridation. Nitridated layer were identified as AlN grains with <1120> preferred orientation, which acted as the nuclearation layers for a-plane GaN growth. Moreover, the qualities improvements were attributed to enhancing grain uniformity and size with 5 min nitridation.