Summary: The flexibility of production ordering systems as medium range production control systems are studied. The changes affecting flexibility, production ordering systems as responses to changes, and the performance measures of flexibility are clarified. On the basis of this framework, mathematical models of production system and production ordering systems are developed to evaluate their flexibility. Two types of changes are considered: demand fluctuation and downtime fluctuation due to machine breakdown, and two models for ‘push-type’ and ‘pull-type’ production ordering systems are investigated. By simulating the models, the amplifications of production quantities and inventory levels as flexibility measures for each type of production ordering system are analyzed and the flexibility compared.