Abstract: As a typical power electronic device, interaction between submodule capacitance, arm inductance and its control system in a modular multilevel converter（MMC） may induce circulating current oscillation. This paper studied the internal circulating current oscillation of an MMC applying impedance analysis. The effects of main circuit parameters, circulating current controllers and controller delay on the internal harmonic resonance characteristics of an MMC were also investigated. The proposed model can be effectively used to guide the internal stability design of an MMC. Also, the proposed model owns clear physical meaning which is convenient to understand and use for engineers. Time-domain simulations in PSCAD/EMTDC verifies the effectiveness and accuracy of the proposed models and analyses.