In this article, a difunctional RAFT agent S, S′-bis (α, α′-dimethylacetic acid) trithiocarbonate (BDAT) was synthesized and used as chain transfer to mediate the reversible addition-fragmentation chain transfer (RAFT) polymerization of methyl methacrylate (MMA) in microemulsion. The influence of polymerization temperature and concentration of RAFT agent on the polymerization were investigated respectively, and the polymerization kinetic constants were calculated. Experimental results showed that the RAFT polymerization in microemulsion exhibited living polymerization characters. The molecular weight of products increased linearly with conversion, the polydispersity indexes remained low value, and the polymerization processes were totally under control with increasing concentration of RAFT agent. In addition, the influence of RAFT concentration on the particle size was investigated by TEM. The TEM results indicated that the particles were highly monodispersed and the particle size increased with increasing concentration of RAFT agent.