Abstract: Modular multilevel converter (MMC) has been widely developed and used in recent years. As an important part in sub-modules (SMs) of MMCs, capacitors play the role of DC support and energy storage. Thus, it is necessary to carry out the research on the sub-module capacitor condition monitoring to ensure the safe and reliable operation of MMC systems. In this paper, an online condition monitoring method was proposed. Firstly, by utilizing this method, multi-point sampling was performed on the voltage of sub-modules within the power frequency cycle, and then information including the IGBT switching signals, modulation ratio, AC side current and voltage, amplitude, and phase is combined to construct voltage and current state equations. Finally, the capacitance value and equivalent series resistance (ESR) value can be obtained by solving the equations. The accuracy and effectiveness of the proposed method are verified by the simulation of the single-phase seven-level MMC system and the hardware-in-the-loop experiment. The simulation results indicate that the maximum absolute errors for the C value and the ESR value are 0.03% and 0.5%. The experimental results show that the maximum absolute errors of C value and the ESR value are 1.5% and 10%, which meets the requirements of submodule capacitor status monitoring.