Due to a series of unique merits, Modular Multilevel Converters (MMCs) have been developing dramatically in the last decade. However, in some applications like offshore power transmission, the MMC modules are subjected to a variety of tough temperature profiles with adverse working conditions; all of these could lead to thermo-mechanical fatigues in the components and joints of the power modules and thereby causing reliability challenges. This study focuses on the lifetime evaluation of MMC based on power cycling and thermal cycling considering the mission profiles in the High Voltage Direct Current (HVDC) application for offshore wind power. The mission profiles as well as the topology of the MMC are first analyzed in order to better understand the loading profiles of MMC. Then the power loss calculation along with the electro-thermal simulation is conducted to uncover the thermal loading of MMC. At last, the lifetime of MMC is studied based on rain flow counting and existing lifetime models for power semiconductors.