Atom transfer radical polymerization is successfully conducted with an organic semiconductor (OS)-based visible light activated photocatalyst via a precisely designed catalytic cycle. The living nature of the polymerization by the OS type photocatalyst is proved by the linearly-increasing molecular weight with consumption of the monomer, the expected molecular weight from MALDI-TOF measurements, and the chain extending reaction, as well as block copolymer synthesis. Benefitting from clear and measurable reductive potential as well as the oxidative potential of the OS-type photocatalyst, the intrinsic demand for "living radical polymerization" is discussed and examined, which indicates the diversity and great potential of OS-type photocatalysts for organic synthesis.