Abstract: JIT (just in time) is an effective mode of optimized purchasing management for supply chain. To implement JIT purchasing, its basic idea and the difference between it and traditional purchasing under the supply chain management are described. Then, a minimization model of total purchasing cost of producer is proposed for JIT purchasing order allocation among multiple suppliers, which shall satisfy a specified percentage of on-time delivery and conform with specific purchasing strategies. Based on the minimum purchasing cost calculated via simulation, a numerical example is given to illustrate the scheme of best order allocation. The sensitivity of purchasing cost to the percentage of on-time delivery is analyzed, and how to study further the problem is predicted.