Imperfections at the regrowth interface in GaP liquid-phase epitaxial growth using the temperature difference method under controlled vapor pressure are investigated by changing the surface treatment and growth conditions. High excess ion density at the interface is observed by the capacitance-voltage (C-V) measurements, while excesses of oxygen, silicon and iron atoms at the interface are observed using the secondary ion mass spectrometry (SIMS). It is possible to decrease the high excess ion density by using a HCl/H2O etching solution and also by controlling the growth rate. Phosphorus vapor pressure treatment reduces the excess of oxygen atoms, but increases the excess of silicon atoms at the interface.