This paper investigates an optimal decision problem in a single-period, two-stage supply chain with capacity reservation contract. At the beginning of the planning horizon, the retailer, who faces stochastic demand, reserves future capacity according to his forecasting of the demand. The supplier then constructs capacity. At the beginning of the selling season, the retailer updates the demand forecasting and places an order. When the retailer's demand is greater than the supplier's capacity, the supplier can meet the retailer's demand by outsourcing. We analyze the optimal decision of each player in both centralized and decentralized systems. Furthermore, under the case in which demand follows a uniform distribution, we obtain the closed-form optimal strategies of each player for both centralized and decentralized systems and conduct numerical studies to reveal additional conclusions. The numerical studies show that the optimal reservation capacity for the retailer and the optimal constructing capacity for the supplier in the decentralized system are both less than the optimal constructing capacity in the centralized system. Furthermore, we also find that the profit loss due to decentralization always exists and increases in indeterminacy.