A new emulsion polymerization process,in which water acted as the dispersed phase and the mixture of methyl methacrylate (MMA) and cyclohexane as the continuous phase,was applied to prepare PMMA resin．It showed that porous PMMA particles composed of aggregated primary particles were prepared by the new polymerization process．The size of these primary particles was smaller than that of the primary particles of PMMA resin prepared by MMA suspension polymerization in the presence of cyclohexane．According to the phase structure of the initial polymerization system．the solubility of PMMA in the mixture of MMA and cyclohexane and the variations of particle size distribution and morphology of PMMA resin,a particle formation mechanism of the new polymerization was pmposed,covering the formation and grouth of primary particles and their coagulation in the dispersed water droplets．