This paper proposes a new sub-module capacitor condition monitoring method for modular multilevel converters (MMCs) based on the DC-side start-up procedure. During this process, the MMC can be simplified into an RC charging circuit, and the sub-module capacitance can be estimated by investigating its relationship with the phase current and sub-module voltage. Several practical issues related to the implementation, advantages, and limitations of the proposed method are presented as well. In order to validate the method, a series of experiments with different sub-module capacitances are conducted based on a three-phase MMC platform. Experimental results confirm the effectiveness and feasibility of the proposed method.