We fabricated GaP liquid phase epitaxial layers with temperature difference method under controlled vapor pressure (TDM-CVP), successively to GaP substrate annealing at growth temperature (1133K) under various phosphorus vapor pressures around the optimum one (150 torr) for stoichiometric crystallization of GaP. Photocapacitance (PHCAP) and photoluminescence measurements have been carried out for characterizing GaP substrate crystals and the LPE layers and for revealing the mechanism of defect formation. Over one order of magnitude decrease of deep level densities in PHCAP has been observed after substrate crystals are annealed. Compared to the layers on substrates without heat treatment, similar tendency of deep level density deduction has been found in GaP LPE layers on heattreated substrates. In photoluminescence measurements, an obvious decrease of 700nm-band intensity from heat-treated substrates, as well as increases of green band intensities from GaP LPE layers on heat treated substrates, have been observed at 77K. Phosphorus interstitial is suggested to be possibly related to the origin of deep level centers in the GaP substrate crystals. It is considered that during the process of heat treatment at growth temperature, under the optimum phosphorous pressure, the defects in GaP substrate crystals reach an equilibrium state, so that their diffusion to the growth layer decreases greatly.