Cooperation is an approach to improve the competitive advantages of a supply chain , and it is also a means of business risk avoiding for the enterprises in the supply chain. A two-echelon supply chain consists of a manufacturer and a retailer is studied , and the demand uncertainty is described by variance coefficient for single2pe2 riod product . It is proved that the cooperative game equilibriumonly exists under the condition that the variance co2 efficient doesn’t exceed certain value. By numerical analysis of influence of demand uncertainty on cooperative game equilibrium and cooperation effects , and by comparing the cooperative with the non2cooperative under given retailing price , cooperation is found to improve the ability of supply chain to deal with market demand uncertainty and busi2 ness risk.