This paper presents a structural model of code sharing among major U.S. domestic airlines and estimates a profit-sharing rule. The profit-sharing rule between partner firms in code sharing is estimated at 0.92, which suggests that the operating carrier acquires around 92% of profits from a round-trip, and the marketing carrier retains 8% as a commission fee. Meanwhile, the economies of code sharing reduces marginal cost, and firms are able to price at higher markups. This implies that demand increases and consumers have larger surplus if code sharing creates new products.