In this paper the electro-parameter of high purity GaAs epitaxial layers and their process conditions are reported. The vapor epitaxial growth is obtained in the Ga-AsCl3-H2 reaction system at high water vapor partial pressure （μ4.1 ×10-6atm） and low deposition temperature （650℃）. The optimum mobility value at 77°K is 21lOOOcm/s.V and its corresponding peak （44.2°K） is 329000cm/s.V. The 77°K mobility values are generally repeated at 120000-200000cm/s.V. And the high pure materials have an outstanding characteristic of low compensative ratio. In this paper

the experimental results are briefly discussed too.