An ultrasound-assisted transesterification process was fabricated in the miscible polycarbonate (PC)/poly(methyl methacrylate) (PMMA) blends to implement in situ compatibilization of PC/PMMA. The effects of sonication time and ultrasonic power on the transesterification process were systematically investigated. Fourier transform infrared spectroscopy (FTIR) measurements showed that ultrasonic oscillation could initiate the transesterification in PC/PMMA blends which heating treatment could hardly initiate. And a higher ultrasonic power and longer sonication time were beneficial to the transesterification process. The H-1-NMR analysis, molecular weight distribution analysis, and gel content calculation indicated that the transesterification process rather than the degradation of PMMA was more favored under sonication. Compared with PMMA, PC was much more prone to break under the application of ultrasonic oscillation. Considering the synergetic effect of polymer degradation and the diffusion of free radical into phase interfaces under ultrasonic oscillation, a potential mechanism of the transesterification process within PC/PMMA blends was proposed at last. POLYM. ENG. SCI., 58:1508-1515, 2018. (c) 2017 Society of Plastics Engineers