Modular Multilevel Converter (MMC) is widely used in flexible DC transmission systems for its great performance in flexibility, stability and availability. Reliability is one of the most important challenges for the MMC consisting of a large number of submodules (SMs). The capacitance in each SM of the MMC is of great importance to the MMC's normal operation. This paper proposes a capacitance monitoring method for the full-bridge SM based MMCs, which can estimate the capacitance in each full-bridge SM with a simple algorithm. The simulation studies with the professional tool PSCAD/EMTDC are conducted with the proposed method. The study results show the effectiveness of the proposed method.