Abstract: We find a kind of surface defects on polished undopcd LEC GaAs wafer (100) by laser light scatter measurement，and prove that kind of defects is coming from micro defects in crystal.And the micro defects has four aggregation centers. Then there is a new way to research the macro property of crystal micro defects.by a serial of qualitative experiment，We get the conclusion that the surface defects we found is the etch pits of arsenide precipitations which exist in the crystal，and we research the distribution regular pattern of arsenide precipitations in crystal along radial and longitudinal direction and the formation principle.

Key words: GaAs, LEC, arsenide precipitation, micro defects, polish wafer, surface defects