A method for determining failure rate and selecting a best burn-in time is disclosed. The method comprises the following steps. First of all, integrate circuits are provided. Then a life-time testing process is performed, wherein a failure rate versus testing time relation is established by measuring the life-time of each integrated circuit under a testing environment, wherein an acceleration factor function also is established under the testing environment. Next a simulating process that uses a testing time function is performed to simulate the failure rate versus testing time relation. Then a transforming process that uses the acceleration factor function is performed to transform the testing time function into a real time function. Finally, an integrating process is performed to integrate the real time function through a calculating region to acquire an accumulated failure rate real time function.