The GaAs epitaxial layers were grown on Si(100) substrates by molecular beam epitaxy(MBE) using the two-step method. The Si(100) substrates were cleaned with different surface cleaning method of vacuum heating, As-beam, and Ga-beam at the substrate temperature of $800^{circ}C$. Growth temperature and thickness of the GaAs epitaxial layer were $800^{circ}C$ and 1 ${mu}m$, respectively. The surface structure and epitaxial growth were observed by reflection high-energy electron diffraction(RHEED) and scanning electron microscope(SEM). Just surface structure of the Si(100) substrate cleaned by Ga-beam at $800^{circ}C$ shows double domain ($2{ imes}1$). RHEED patterns of the GaAs epitaxial layers grown on Si(100) substrates with cleaning method of vacuum heating, As-beam, and Ga-beam show spot-like, ($2{ imes}4$) with spot, and clear ($2{ imes}4$). From SEM, it is found that the GaAs epitaxial layers grown on Si(100) substrates with Ga-beam cleaning has a high quality.