A GaAs transmission photocathode structure was prepared.The variations of the cathode photosensitivity and mean transverse energy through the course of typical activations were measured systematically in the photocathode activation and imaging system. The GaAs(Cs,O) photocathode surface was examined by scanning electron microscope. It was concluded that the transverse energy of photoelectrons emitted from GaAs(Cs,O) photocathodes, which is largely independent of the (Cs,O) activation layer, is determined by the GaAs lattice temperature, photocathode surface roughness and multiple scattering processes in the band bending region.