Just in Time (JIT) production systems are introduced for production and supply of goods and services with minimum inventory. The goal of this paper is present a sample of system dynamics modeling application for analysis and improvement of JIT system's behavior. In this paper relations between demand of products, quality of produced goods, inventory, maintenance of machines and facilities, production rate and supplier's efficiency are investigated in a dynamic system. In the first section problem definition, dynamic hypothesis, causal diagram and flow diagram of system are discussed. Afterwards behavior of the model is inspected from different views by developing a dynamic model.