We Successfully developed a transmission-type photocathode, and a high polarization (90%) with a super-high brightness(1.3 x 10(7) A cm(-2) sr(-1)) of electron beam was achieved. In this research,we found that the polarization of electrons from the GaAs-GaAsP superlattice on the GaP Substrate was lower than that from the same superlattice oil the GaAs substrate, and that a GaAs inter-layer deposition on the Gap substrate recovered that from the GaAs-GaAsP superlattice. Mechanism of the polarization degradation was investigated from a viewpoint of strain-relaxation process in the GaAsP buffer layer that was adopted between the superlattice and the substrates. The buffer layer with in-plain tensile strain was relaxed by inducing cracks, but that with in-plain compressive strain was relaxed by inducing dislocations. The dislocations may disturb the spin-polarization of electrons more effectively than the cracks do. (C) 2008 Elsevier B.V. All rights reserved.