Multivariate analysis was applied to the polymerization data of a thermal expansion microcapsule to clarify the operating conditions affording the steadiest particle diameter control. As a result, among the factors controlling the particle diameter of a thermally expandable microcapsule, the amount of the dispersing agent, the stir power in the preliminary mixing tank and the curing tank, and the liquid physical properties prescribed for polymerization were incorporated into a statistical model. It was shown that, after a thermal expansion microcapsule is polymerized, the mean particle size can be reduced by increasing the amount of dispersing agent and the stir power of the preliminary mixing tank and the curing tank, which is a process factor. It was also shown that although the physical properties of the raw material influenced the liquid drop diameter immediately after passage through the geostationary type decentralization machine, the polymerization was controlled by the amount of dispersing agent and the stir power in the curing tank.