The effects of varying As4/Ga flux ratior, substrate temperature and growth rate on the deviation from stoichiometry in GaAs grown by MBE have been investigated. The deviation from stoichiometry was directly measured by detecting intensity variation of a weak quasiforbidden (200) reflection of x rays. Using this new method, we obtained the following results: in the regions ofr2.0, higher densities were detected in the As lattice plane. The behavior forr<1.0 is contrary to the expectation of decrease of As. A possible explanation for this higher reflection inr<1.0 is that some Ga atoms may exist at interstitial site in the As lattice plane. It is also seen that densities of atoms in the As lattice plane decrease with increasing substrate temperature. In correspondence with the results that the growth condition with flux ratior=1.4 minimizes the deviation from stoichiometry, it is found that GaAs films grown under the conditionr=1.4 show the highest quality of the electrical and optical properties.