Thin films of GaAs single crystal were prepared successfully on GaAs (100) substrates by molecular beam epitaxy using an ultrahigh vacuum system with effusion cells and a quadrupole mass analyzer. The dependence of the quality of the GaAs film on the substrate temperature, Ts, was investigated in detail at a constant Ga and As2 arrival rate. The epitaxial process was studied in situ with a high energy electron diffraction (HEED) system. The analysis and characterization of the prepared GaAs films were made with a scanning electron microscope, an ion microprobe analyzer, an ESCA photoelectron spectrometer and photoluminescence measurements. These measurements indicate that GaAs films grown at 480\lesssimTs\lesssim530°C by molecular beam epitaxy have smooth surface and nearly the same quality as bulk crystal.