The effect of LT GaAs on the effective barrier height of the epitaxial Al/GaAs Schottky contact was investigated for the first time by inserting a thin LT GaAs layer (50 ∼ 500Å) between the in situ deposited Al film and conventional MBE GaAs epitaxial layer. The activation energy plot of saturation current for the devices showed that the effective barrier height exhibits a dependence on LT GaAs thickness and reaches a saturated barrier height when the LT GaAs layer exceeds a critical thickness. Compared to the samples which had no LT GaAs layer, the effective Schottky barrier height was decreased from 0.79 eV to 0.35 eV for the n-GaAs samples, and increased from 0.55 eV to 0.72 eV for the p-GaAs samples. The Schottky barrier height modification achieved by LT GaAs is tentatively explained in the terms of a bulk Fermi level pinning model. The work described here suggests that LT GaAs can be used as a defect source with controlled thickness to study defect associated phenomena such as Schottky barrier height modification.