Monitoring capacitance is a key content for the modular multilevel converter (MMC) to acquire health status of capacitors in submodules (SM). This thesis advances a capacitance monitoring approach for MMC’s SM specifically tailored for use in photovoltaic inverter, based on the simple and cost-effective pre-charge method. Under this method, the MMC’s SM capacitor could be charged one by one with one nominal SM voltage, the SM capacitance can be evaluated by studying its relation with the charging current and SM voltage. By contrast with the traditional capacitance monitoring means, the presented approach with the benefits of not disturbing or impacting the normal converter operation, does not involve additional measurement circuits, sensors and complex control algorithms. By execute the experimental studies, the outcome verifies the availability of the presented method.