Polymerization of methacrylonitrile (MAN) with methylaluminum porphyrin (1) as initiator proceeded rapidly in the presence of methyaluminum bis(2,6-di-tert-butyl-4-methylphenolate) (3). The living polymer of methyl methacrylate (MMA) with a (porphinato)aluminum enolate growing terminal (2), prepared by the polymerization of MMA with 1, also brought about the polymerization of MAN in the presence of 3, affording the corresponding block copolymer with M(W)/M(n) ranging from 1.1 to 1.3. The living nature of polymerization was confirmed by a delayed sequential two-stage polymerization of MAN initiated with 2 (living prepolymer of MMA on aluminum porphyrin). Pyridine instead of 3 was also effective for the rapid block copolymerization of MAN from 2 to give the block copolymer of controlled molecular weight. Copolymerization of MAN and MMA initiated with 1 proceeded more rapidly in the presence of 3 or pyridine than in the absence. In the presence of 3, the consumption of MAN was much more accelerated than that of MMA, while in the presence of pyridine, the consumptions of MAN and MMA were almost equally accelerated. The poly(MMA-b-MAN) obtained by the block copolymerization of MAN from the living prepolymer of MMA (2) was found to form micellar aggregates in organic solvents such as CHCl3 and THF, depending on the composition and block length, as well as the concentration and temperature. Furthermore, a microphase separated structure was clearly observed by transmission electron microscopy for the block copolymer membrane cast from CHCl3/THF.