Thruster break for aerogenerator performs a role that is stopping the rotor in case of emergency such as strong wind, system abnormality or maintenance check. In this study, failure analysis and test evaluation on the thruster break for aerogenerator are proposed, and the entire process to improve reliability of the product through design improvement is presented. The typical failure case is fatigue and wear, and failure cause on these are identified and improvement plan is presented. Lastly, reliability improvement is established to analyse test results before after the life test.