The effects of compatibilizer on the morphological, thermal, mechanical, and rheological properties of poly(methyl methacrylate) (PMMA)/poly(N-methyl methacrylimide) (PMMI) (70/30) blends were investigated. The compatibilizer used in this study was styrene-acrylonitrile-glycidyl methacrylate (SAN-GMA) copolymer. Morphological characterization of the PMMA/PMMI (70/30) blend with SAN-GMA showed a decrease in PMMI droplet size with an increase in SAN-GMA. The glass-transition temperature of the PMMA-rich phase became higher when SAN-GMA was added up to 5 parts per hundred resin by weight (phr). The flexural and tensile strengths of the PMMA/PMMI (70/30) blend increased with the addition of SAN-GMA up to 5 phr. The complex viscosity of the PMMA/PMMI (70/30) blends increased when SAN-GMA was added up to 5 phr, which implies an increase in compatibility between the PMMA and PMMI components. From the weighted relaxation spectrum, which was obtained from the storage modulus and loss modulus, the interfacial tension of the PMMA/PMMI (70/30) blend was calculated using the Palierne emulsion model and the Choi-Schowalter model. The results of the morphological, thermal, mechanical, and rheological studies and the values of the interfacial tension of the PMMA/PMMI (70/30) blends suggest that the optimum compatibilizer concentration of SAN-GMA is 5 phr.