Change of the surface density of the midgap level (EL2 or EL0) in bulk GaAs by heat treatment was studied on horizontal Bridgman (HB) and liquid encapsulated Czochralski (LEC) grown GaAs by changing the capping materials (Si3N4, SiO2, GaAs) and the ambient (H2, vaccum). Decrease of the surface density of the midgap level was prevented on HB GaAs by capping with Si3N4, SiO2or GaAs, whereas it could not on LEC GaAs when it is capped with Si3N4or SiO2. Emission rates of the midgap level in LEC GaAs changed by the heat treatment but it did not for HB GaAs. These results support that the midgap levels in HB and LEC GaAs are due to different point defects (EL2 or EL0).