Native defects in Si-doped, Zn-doped and undoped GaAs grown by horizontal Bridgman (HB) method and molecular beam epitaxiy (MBE) were studied by a monoenergetic positron beam. Positron lifetime spectra and Doppler broadening profiles were also measured by using energetic positrons. It was found that monovacancies were usually found in Si-doped HB-GaAs, however, divacancies were created in a specimen with low impurity concentration. For Zn-doped HB-GaAs, interstitial clusters were found to be major type of defects. The high concentration of monovacancies and of divacancies coexist in heavily Si-doped MBE-GaAs. These vacancy-complexes were found to act to reduce the concentration of free carriers.