On the basis of strict mathematical description about the assurance test, the optimal lower confidence limit and upper confidence limit are found out by the method of arranging order in the sample space. 在对MTBF(平均无故障时间间隔)保证试验进行严格的数学描述的基础上,利用样本空间排序法找出了MTBF在一定意义下最优的置信下限和置信上限(UCL)。