In this paper, reliability-oriented switching frequency analysis for Modular Multilevel Converter is investigated. In the medium voltage and high voltage direct current applications, the switching frequency should be low enough to avoid the converter from high switching power losses. In the other hand, choosing the switching frequency very low will impose higher ripple over the capacitor and will increase the THD of the output current. The result of this paper shows that although less semiconductor losses are incurred when operating at lower switching frequencies, the increase of the voltage ripple across capacitors and IGBTs can detrimentally affect the reliability of the converter, illustrating one of the key tradeoffs when designing the MMC.