Effects of controlled arsenic vapor pressure thermal treatments on deep levels of liquid-encapsulated Czochralski (LEC) GaAs, especially on the midgap donor EL2, are investigated. Undoped LEC n-type conductive and semi-insulating GaAs crystals are annealed in vacuum or under As vapor pressure. From depth profiles of EL2 concentration, it is confirmed that the thermal conversion on semi-insulating GaAs is caused by the reduction of EL2 concentration and the introduction of native acceptor (VAs). A three-step model for EL2 out-diffusion is proposed. The formation of EL2 occurs during the cooling process at a relatively low temperature of around 650 °C.