Abstract: Joint purchasing tends to make the order quantity multiply, and enjoys the suppliers' price discount more easily, so the retailers favor this purchasing mode. During the period of holding, the ameliorating items will cause ameliorating phenomenon, and lead to ameliorating cost which affects the purchasing decision of the retailers. Moreover, it's common that the suppliers provide non-instantaneous replenishment mode in our daily life. Considering two-echelon supply chain of ameliorating items consisting of a single supplier and multiple retailers, the suppliers provide price discounts and supply in non-instantaneous replenishment mode. The function of cost per unit time of independent purchasing and joint purchasing for the retailers are set up separately,the cost per unit time of the two purchasing modes are compared in this paper, and the suitable condition that the joint purchasing is superior to the independent purchasing is gotten.
In joint purchasing, it must be assured that the cost per unit time allocated by the purchasing alliance cannot be higher than the cost per unit time of retailers' dependent purchasing. Otherwise, the purchasing alliance will be disrupted. The alliance cost of joint purchasing is treated as the allocation object, multiple persons cooperative game theory is applied to transforming the cost allocation of joint purchasing problem into multiple persons cooperation game problem, and the model of cost allocation is established based on the Least Core method. Then the process of cost allocation is demonstrated according to a numerical example, the sensitivity analysis of the ordering parameters and the cost parameters for the net ameliorating rate are given, and the allocation results of Shapely Value Method, Simplified Minimum Costs-Remaining Savings Method, Two Programming Method and Least Core Method are compared. The research conclusion will provide a theoretical reference for the ameliorating items' joint purchasing and cost allocation.

Key words: ameliorating items, non-instantaneous replenishment, joint purchasing, cost allocation