On the assumption that the input voltage can be replaced by its RMS value and the frequencies of the inductor current and the output voltage are much lower than the switching frequency of the converter, the small-signal model of Boost PFC converter is constructed and the medium-frequency oscillation in Boost PFC converter is studied. The underlying mechanism of this nonlinear phenomenon is analyzed and the stability conditions are also given. These obtained results indicate that this type of nonlinear phenomenon is different from the fast- and slow-scale instabilities, and the main characteristic of this type of nonlinear phenomena is that its oscillation frequency lies between the line frequency and the switching frequency. Finally, an experimental circuit is designed, and its experimental results agree very well with the simulation results.