Scattering defects and microscopic PL in MBE-grown GaAs epitaxial layers on semi-insulating (SI) substrates with different dislocation density were studied by laser topography. The epitaxial layer on a lower-dislocation-density substrate (EPD = 4E3 cm(-2): grown by VB method) has about one order fewer scattering defects than that on a higher-dislocation-density substrate (EPD = 3E4 cm(-2): grown by LEC method), in which scattering defects were observed with cellular patterns like dislocation distribution in the substrate. PL characteristics are the same in the two epitaxial layers. The effects of these scattering defects on power devices are discussed.