The effects of interstitial Mn (MnI) atoms on the electronic and magnet ic properties of (Ga,Mn)　As are studied within the pseudopotential plane_wave f rame work based on the density functional theory. Numerical results show that MnI atoms are donors in GaAs. The Mn atoms that substitute for Ga sublattices (MnGa) and MnI are antiferromagnetically coupled. The static electrical interaction makes the MnGaand MnI atoms tend to form MnGa_MnI pairs. The MnI atoms not only compensate the holes, and reduce the hole density of (Ga,Mn)As, but also deactivate MnGaatoms. The existence of MnI atoms are very disadvantageous for obtaining (Ga,Mn)As samp les having high Curie temperatures.