Thin films of GaAs on Si were grown by molecular beam epitaxy, which involves the solid phase epitaxial (SPE) growth of the amorphous GaAs buffer layer. A Rutherford backscattering minimum channeling yield of ∼9.4% has been obtained for a 0.8‐&mgr;m‐thick GaAs film. Cross‐sectional transmission electron micrographs and reflection high‐energy electron diffraction results have revealed that misfit dislocations are mostly confined to what used to be the buffer layer of ∼300 nm in thickness, and that the microtwins (and/or stacking faults) are mostly originated from the GaAs/Si interface and are generated during SPE growth.