Using molecular beam epitaxy assisted by an electron beam irradiation with a beam energy of 50 keV at an angle of incidence 5°, we have successfully grown high quality GaAs layers on untilted (001)Si substrates. Raman spectra show that this technique reduces crystalline imperfections usually found in GaAs layers grown (001)Si substrates by a factor of 20, clearly demonstrating the effectiveness of this technique. Typical low‐temperature photoluminescence spectra of undoped GaAs on Si contain four features at emission energies of 1.503, 1.488, 1.471, and 1.434 eV. The full width at half maximum (FWHM) of the bound excitonic peak with heavy holes is 2.8 meV, which can be one of the narrowest ever reported. The incident electron beam is expected to play several roles, such as cleaning Si surfaces by removing carbon/oxygen residuals, enhancement of migration rates of adatoms on the growing surface, and acceleration of the single‐domain formation during the growth.