Abstract: In this article, according to the optimal criterion of minimizing the asymptotic variance of the maximum likelihood estimation of the logarithm of the 100pth-percentile in the lower tail of the failure-time distribution at work conditions, methods and guidelines are given for planning two-factor time step-stress accelerated life test experiments (ALT) for models in which there is no interaction between the factors when life-time of the tested products is exponential distributed at constant testing conditions. The design given here is also proved to be D-optimal.