Abstract: The credit sales can enhance the competitiveness of enterprises, expand business scale, and help sales enterprises to ease the financial pressure. It has a great influence on the operation efficiency of the supply chain, and has become one of the important research topics in the field of supply chain management. The ameliorating items are a kind of ubiquitous goods, but the problem of joint purchasing for ameliorating items has not gotten enough attention. In a two-echelon supply chain of ameliorating items consisting of one supplier and multiple retailers, the supplier provide permissible delay in payments as a means of credit sales. The function of total cost per unit time of independent purchasing and joint purchasing for the retailers are built up respectively, and the optimal order cycle and minimum total cost per unit time of retailers under two purchasing modes are obtained. According to the relationship between the optimal order cycle and the delayed payment period, the functions of minimum total cost per unit time of the two purchasing modes are compared, and the conditions that joint purchasing is better than independent purchasing are got. On this basis, the minimum total cost per unit time of joint purchasing is treated as the allocation object, and multiple persons cooperative game theory is applied to transforming the cost allocation of joint purchasing problem into multiple persons cooperation game problem. After providing some basic properties of the game, a cost sharing method is designed based on the core of the game. Finally, the numerical example and sensitivity analysis are used to illustrate the theoretical results, and some managerial implications are put forward. The research can provide practical guidance significance for the enterprises who engaged in the production and sale of ameliorating items.

Key words: joint purchasing, ameliorating items, credit sales, permissible delay in payments, cooperative game