A method suitable for engineering application is presented to determine the minimum sample size of missile-borne electromechanical products in storage environmental test.Firstly, PAVA, a method for processing “reversal” data based on isotonic regression, is used for data pre-processing.Then, the lifetime distribution model is established by using the storage reliability evaluation method combining minimum chi-square estimation with goodness-of-fit test.Finally, the relative deviation of estimator is used as the precision index, the minimum sample size under given precision requirement and confidence level is calculated out.A formula indicating the relation among sample size, precision index, confidence level and distribution parameters is derived.The result shows that the minimum sample size is fit for engineering application.