This paper presents the first demonstration of blue-ultraviolet avalanche photo-diode (APD) operation using II-VI wide bandgap semiconductor ZnSe, grown on GaAs substrate by molecular-beam epitaxy. With improving crystal quality on macroscopic defects as well as an effective mesa-etching technology, the p(+)-n structure ZnSe diode has shown a distinct APD operation with a large avalanche gain of G = 50 under high reverse bias of 27 V at room temperature.