Newly designed cyclometalated iridium phosphors bearing 2,3-diphenylquinoxalines were characterized to provide highly efficient and vivid-red emitting materials for electrophosphorescent organic light-emitting devices. Excellent quantum efficiencies for photoluminescence (PL) within a range 50-79% were observed in dichloromethane solutions at room temperature. A greatly improved PL decay lifetime of 1.1 µsec was also observed in CBP coevaporated film. Luminescence peak wavelengths of the phosphors lay within a preferable range 653-675 nm in evaporated films. The most vivid-red electroluminescence with 1931 CIE chromaticity coordinates of (x=0.70, y=0.28) was successfully attained.