Polymerizations were carried out in a stainless steel capillary of 500 Mm inner diameter as a microreactor. Polystyrene of number average molecular weight of 10000 was synthesized during radical polymerization with 2,2'-azoisobutyronitrile (AIBN) as initiator without plugging the capillary Copolymerization of styrene (St) and methyl methacrylate (MMA) was carried out in capillary-type microreactor with T-type joint. St and MMA monomers were fed into branched capillaries separately and AIBN initiator was well mixed with St or MMA before polymerization. Two-phase flow was formed at the entrance region of the microreactor, and the difference in viscosity between the each phases increased with the degree of polymerization. Flow behaviors of two phases with different viscosities were also investigated. When AIBN initiator was mixed with MMA, polymerization of MMA caused an increase in viscosity and a prolongation of residence time. As a result, polymer composition could be significantly altered in two-phase microsystem compared to that synthesized in homogeneous reaction system.