At present, when the enterprises that have implemented the SCM chose co - vendors, all kinds of methods have been employed to appraise them. These results are always influenced by the members' subjective attitude at any rate. Based on the fact that the cost is the most important factor when choosing, and according to the requirement of the enterprise on the supply chain, the authors research the cost's component on supply chain. The cost includes: quality cost, price cost, stock cost, OOS cost, transportation cost and other cost. By calculating the products'practical cost synthetically , the enterprises could choose the vendors whose cost is cheapest as their co - vendors. The calculate cost is an objective and feasible method to choosing vendors reasonably. Finally, some examples and related discussion are given.