Reliability tests are mandatory to evaluate new products prior to their release. The proper determination of a reliability test plan is crucial because an erroneous test plan can be very costly and misleading. This paper describes a probability-based method of designing a reliability demonstration test plan using both field customer usage and historical bench life test data. Statistical distribution analysis, Monte Carlo simulation (MCS) technique, and zero-failure test method are integrated into the probability-based method to create test plans that can more accurately evaluate product reliabilities for the required product service life using a small number of test samples.