We propose a new technique for the growth of high-quality III-nitride semiconductors with ultra-flat surface morphologies and low dislocation density. The key-point of the technique is the use of vicinal sapphire (0001) substrates for the growth. We use plasma-assisted molecular beam epitaxy to grow the films. As a result, ultra-flat surface with mono-layer steps is achieved using a 0.5^o vicinal substrates. Furthermore, the dislocation density in the GaN film grown on a 2.0^o vicinal substrate is greatly reduced to the order of 10⁸/cm², which is more than one order lower than that grown on a usual just-oriented substrate. The technique provides a new chance for the high performance of III-nitrides-based optical and electronics devices.