Zinc blende and wurtzitic GaN films have been epitaxially grown onto (001)Si by electron <m1;&1p>cyclotron resonance microwave plasma‐assisted molecular beam epitaxy, using a two‐step growth process. In this process a thin buffer layer is grown at relatively low temperatures followed by a higher temperature growth of the rest of the film. GaN films grown on a single crystalline GaN buffer have the zinc blende structure, while those grown on a polycrystalline or amorphous buffer have the wurtzitic structure.