The main objectives of this thesis were to analyze the risks of HV circuit-breakers, develop the probabilistic models, breakdown the cost structure and establish the improved maintenance strategies. Normally, HV circuit-breakers have been maintained by using manufacturers’ guidelines and experiences of operators. It is not well proved that such maintenance programs are effective regarding costs and performance. Although there are many literatures introducing the optimal maintenance for HV circuit-breakers, most of them propose only the ideas and mathematical models without the reference from the failure database. In this context it was challenging to design and investigate reliability and maintenance strategies in connection with the failure database. This thesis was constructed of three main parts to handle such problems. In the first part, the failure modes and effects analysis method was applied to determine the risks of components of HV circuit-breakers. The second part was to develop the probabilistic models to investigate the reliability of HV circuit-breakers and failure development. In the last part the cost structure of HV circuit-breakers was determined. In addition, the optimal maintenance strategies during useful and wear-out period were established. The failure database of HV circuit-breakers collected by the Institute of Power Systems, Darmstadt University of Technology had been implemented in all parts.