Partially ionized molecular beam epitaxy (PI‐MBE) is a useful method for growing heavily doped epitaxial films. Heavily As‐doped Si films grown by Si PI‐MBE were investigated. Crystalline quality of the films was examined by transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS), and Hall effect measurement. It was revealed that the electron Hall mobility of the films grown at 850°C by PI‐MBE was nearly the same as that of bulk Si, and that the electrical activity of doped As atoms was superior to that of the films grown by neutral Si MBE. A location of doped As atoms in the crystal lattice of films was estimated by comparison of the electrical activity with the RBS yield. The fraction of substitutional As atoms depended on a doping level and a substrate temperature.