This Master Thesis aims to perform a Risk Assessment of an On Orbit Servicing mission based on space tug concept during a conceptual design phase. FMECA and FTA methods are used as complementary techniques in order to identify all critical components and events during on orbit operations. After a brief introduction about history and definition of risk analysis, a literature review about on orbit servicing is developed. It focuses on three aspects: historical ancestors, modern technological demonstration missions and servicing opportunities and economic feasibility. After that, the space tug design concept is described as a derivation from existing projects with few additions.