A new class of domain morphologies in diblock copolymer systems, i.e. a helical domain morphology, was found in a three component mixture: of polystyrene-block-polyisoprene diblock copolymer, homo-polystyrene, and homo-polyisoprene. An intuitive explanation of such a domain morphology is given, which predicts that this helical morphology can be observed only in a non-equilibrium state.