This paper analyses and compares the performances of two types of production ordering systems for unbalanced serial production systems. One is the sequential ordering system, which is modeled on the Kanban systems, and the other is the concurrent ordering system. Queueing network models of the production ordering systems for 5 stages serial production systems are constructed, and the performances are investigated by simulation experiments. As a result, it is clarified that the concurrent ordering system is more adjustable to the imbalance of production processes.