Microsuspension conventional radical polymerization (ms CRP) of methyl acrylate (MA), which is more hydrophilic monomer than methyl methacrylate (MMA), was carried out at different benzoyl peroxide (BPO) concentrations (0.8-8.0 wt% relative to MA), gave not similar micrometer-size poly(MA) (PMA) particles as MA droplets but submicrometer-size ones. That is, emulsion polymerization proceeded in the ms CRP systems. When microsuspension iodine transfer polymerization (ms ITP) with CHI3 as chain transfer agent was carried out at 8.0 wt% BPO (relative to MA), at which micrometer-size poly(MMA) particles were successfully synthesized without submicrometer-size byproduct particles in a previous particle, only submicrometer-size PMA particles were formed, too. However, the percentage of the byproduct particles obviously decreased with a decrease in the BPO concentration, resulting in zero practically at 0.8 wt% BPO, where number-average molecular weight of the micrometer-size PMA particles was nearly equal to theoretical one and polydispersity was comparatively narrow (M-w/M-n = 1.43). Such experimental results were presumed based on the kinetic simulation study. Above results support the validity of the "radical exit depression (RED) effect" proposed and indicate a decisive effectiveness of ms ITP to improve a long-standing problem of ms CRP of "hydrophilic" monomer.