Nonequilibrium transport of carriers in bulk GaAs under very high electric fields is investigated by time-domain terahertz (THz) emission spectroscopy. It is found that the initial peak height of THz emission waveforms (ΔETHz), which are corresponding to the acceleration of electrons in the Γ valley, gradually increases with increasing bias electric fields F0, for F0<50 kV/cm and saturates at ～1 THz above 50 kV/cm. The experimental results show that the effective acceleration mass of electrons under high electric fields significantly increases with increasing fields (30 times at 300 kV/cm than low bias electric fields), most likely due to strong band mixing under very high fields.