Two formulae to give the mean and the standard deviation of job flow-times are derived from the machine number, the load ratio, the required number of operations, and the mean and the standard deviation of processing times, which specify the characteristics of a job shop. On the basis of the two formulae, a method of due-date assignment which contains a due-date adjustment factor is proposed. The assignment method is combined with the sequencing procedure to construct the total scheduling system for reducing job tardiness. Two new dispatching rules are presented as the second phase of the system. The experimental comparison shows that the efficiency of the proposed systems can be better than the conventional scheduling systems