Increasing research activity on GaMnAs gave in recent years considerable improvements of it's magnetic and electrical properties, in particular significant increase of paramagnetic-to-ferromagnetic phase transition temperature (Tc), from 110 K to 150 K (170 K in some particular case). Even though the methods of fabrication of GaMnAs with the highest Tc are more or less established and repeated by many research groups, it is not fully understood what lies behind the empirical procedures rendering GaMnAs the highest Tc. It is generally accepted that the control of structural defects present in GaMnAs in large concentrations is a key factor for it's magnetic and transport properties, though the overall picture of GaMnAs defect structure is still quite fuzzy. As is well known in the case of LT GaAs, the use of low growth temperatures during Molecular Beam Epitaxial deposition of GaMnAs thin films implicates existence of variety of different structural defects such as As antisites, Ga and As vacancies, As interstitials etc.. Addition of Mn into LT GaAs makes the situation even more complicated, since Mn creates new defects states. At present the main interest has been addressed to Mn interstitials reported to be present in GaMnAs, before, mainly As antisites were concerned. Since both types of defects: Mn_I and As_Ga are present in GaMnAs in large concentrations (of the order of 0.5% - 2%), they both have to be taken into account to explain GaMnAs properties. In this work we demonstrate how to control independently both types of defect and what is their influence on structural, magnetic and transport properties of GaMnAs.