Iodine transfer polymerization (ITP) with CHI3 as transfer agent and reversible chain transfer catalyzed polymerization (RTCP) with N-iodosuccinimide (NIS) as catalyst of methyl methacrylate (MMA) were successfully applied to aqueous microsuspension systems (respectively, microsuspension ITP and microsuspension RTCP). Both microsuspension ITP and RTCP proceeded smoothly without induction period and with a controlled/living manner. Polydispersity index (PDI, M-w/M-n.) of the microsuspension RTCP was smaller than that of the microsuspension ITP, and it was larger than RTCP in a bulk system (bulk RTCP). The difference seems to be based on that NIS would partly react with water dissolved in the MMA phase, and formed succinimide, which is a low controllable catalyst in comparison with NIS. With decreasing particle (monomer droplet) size, the polymerization rate increased, and a reasonable control was maintained throughout the polymerization based on the segregation effect.