Block copolymerization of styrene and methyl methacrylate by photochemical initiation was carried out successfully in the water phase with a magnetic field. Poly(ethylene glycol) (PEG) with a naphthyl group at one end and a benzoyl benzyl group at the other end [GRAPHICS] was used as triplet sensitizer. The block copolymerization is dependent on the magnetic field and the concentration of PEG sensitizer. It was found that a magnetic field of 0.035 T is sufficient to influence block copolymerization proceeding in the micelles formed by the hydrophilic PEG chain and hydrophobic sensitizer end groups. The molecular weights of polystyrene-block-poly(methyl methacrylate) are in the range of 10(5)-10(6) and the molecular weight distribution is about 1.61-1.74.