Emulsion polymerization of vinyl acetate with a compartment reactor consisting of 3 well-mixing compartments has been investigated. A step response method revealed that the mixing of this reactor is characterized by a 4 CSTRs-in-series model. Higher conversion of the vinyl acetate monomer is obtained by the compartment reactor than by a single CSTR since the restricted axial dispersion suppresses the effluent of unreacted monomer. The oscillation of the mean particle size induced by the co-presence of small and large particles is inhibited due to narrow residence time distribution of particles, and stable particle size distributions are obtained under steady state operation. Back-mixing causes polymerization with short mean residence time in the first compartment. Since there is little back-mixing, polymerization occurs in the first vessel even when the mean residence time of feed is so low that the polymerization did not occur in a single CSTR. The compartment reactor can be a possible method to convert the batch process into a continuous process.