A single-crystalline epitaxial film of GaAs has been grown on Si using a gs assisted-ionized vapour beam eptaxial technique. The native oxide layer on the silicon substrate was removed at 550$^{circ}C$ by use of an accelerated arsenic ion beam, instead of a high-temperature desorption. During the growth the substrate temperature was maintained at 550$^{circ}C$. Transmission electron microscopy and electron diffraction data suggest that the GaAs layer is an epitaxially grown single-crystalline layer. The possibility of growing device quality GaAs on Si is able demonstrated through fabrication of GaAs MODFET on Si substrates.