The microcellular injection molding (commercially known as MuCell) of in situ polymerization-modified PET (m-PET) was performed using supercritical nitrogen as the physical blowing agent. Based on the design of experiment matrices, the influence of operating conditions on the mechanical properties of molded samples was studied systematically for two kinds of m-PETs, namely, n-m-PET and m-m-PET synthesized using pentaerythritol and pyromellitic dianhydride (PMDA) as modifying monomers, respectively. Optimal conditions for injection molding were obtained by analyzing the signal-to-noise (S/N) ratio of the tensile strength of the molded samples. The specific mechanical properties, especially the impact strength, of the microcellular samples under those optimal conditions increased significantly. Scanning electron microscope analyses showed a uniform cell structure in the molded specimens with an average cell size of around 35 mu m. The m-m-PET modified with PMDA generated a slightly finer cell structure and a higher cell density than the n-m-PET. POLYM. ENG. SCI., 54:2739-2745, 2014. (c) 2013 Society of Plastics Engineers