Epitaxial Si layers with low dislocation density were grown on GaAs substrates by pyrolysis using disilane. The dislocation density evaluated from SECCO etching (HF:H2SO4:K2Cr2O7=100 cc:50 cc:2g) was less than 104/cm2, which is lower by two orders of magnitude than that of the GaAs layer on Si substrates. We found that the electron mobility of Si layers on GaAs substrates could be raised by decreasing the growth rate and a high mobility value the same as that of a homoepitaxial layer was obtained, although the Si layers were contaminated by an arsenic impurity of 1018cm−3.