This paper derives two formulae to calculate the average inventory yielded by fixed interval withdrawal Kanban and supplier Kanban, respectively, in a just-in-time production system. By using the formulae, another two formulae to determine the minimum number of Kanbans to be required for the fixed interval withdrawal Kanban and the supplier Kanban are also shown. An algorithm to obtain the optimal order interval is proposed to minimize the total operation cost composed of the inventory cost and the withdrawal cost for the Kanban system