Abstract: The failure reason and form process of a microstrip ferrite isolator by failure analysis is introduced in detail．The effective solution is found and the failure reason is properly located after experiments．All possible cases must be analyzed to locate the failure reason．The real failure reason can be found and the improved solution can be reliable or effective after enough test ＆validation and practical validation of a period of time．Quality control is very important．Therefore，it should be clear how to perform quality control about the similar problems from the real situation．