In this paper, we illustrate the method of designing a group-sequential randomized clinical trial based on the difference in restricted mean survival time (RMST). The procedure is based on theoretical formulations of Murray and Tsiatis (1999). We also present a numerical example in designing a cardiology surgical trial. Various practical considerations are discussed. R codes are provided in the Supplementary Materials. We conclude that the group-sequential design for RMST is a viable option in practice. A simulation study is performed to compare the proposed method to the Max-Combo and conventional log-rank tests. The simulation result shows that when there is a delayed treatment benefit and the proportional hazards assumption is untrue, the sequential design based on the RMST can be more efficient than that based on the log-rank test but less efficient than that based on the Max-Combo test. Compared with Max-Combo test, the RMST-based study design yield coherent estimand, statistical inference and result interpretation.