Diluted magnetic semiconductors GaFeAs were grown by molecular beam epitaxy and characterized. Ga1-xFexAs ternary alloys were obtained at the growth temperature T-s = 200 degreesC ranging from x = 0.005 to 0.03. The effects of thermal treatment on behaviors of defects, affecting to the magnetic properties of GaFeAs layer were particularly elucidated. As-grown samples were annealed at temperatures varying from 400 to 600 degreesC. From the measurement of double crystal X-ray diffraction, we observed Fe-related peak which shifted to a higher diffraction angle as the Fe content increased, indicating that the lattice constant decreases with increasing Fe content. In contrast, above the annealing temperature 500 degreesC, the lattice constant becomes smaller than that of GaAs. Using the deep level transient spectroscopy, various defects in GaFeAs layer were observed and identified in conjunction with magnetic properties. (C) 2002 Elsevier Science B.V. All rights reserved.