The relationship between the mechanical properties and morphology of polyhexamethylene carbonate diol (PHMCD) toughened epoxy resin was investigated. The parameters describing the morphology (e.g., average diameter of particles and volume fraction of dispersed phase, etc.) were determined through SEM observation. It was observed that the particle size distribution changes from a unimodal distribution to a bimodal one and then tounimodal again with an increase in curing temperature. When it was cured at 120℃, an epoxy resin with a bimodal distribution of rubber particle size was obtained. The mechanical properties, especially the impacting strength, of the resin are the best because of the synergistic effect of the PHMCD particles.