We studied the initial layer of GaAs films grown on Si substrates by a two‐step metalorganic chemical vapor deposition method, in which an initial GaAs layer is grown on a Si substrate at low temperature. It was found that the V/III ratio during the growth of the initial layer plays a key role in improving the quality of GaAs film on a Si substrate. The initial GaAs layer grows two dimensionally as a continuous film when it is grown at a low V/III ratio.