In view of the fact tht many scholars have studied the effect of procut modurity and productin policy on Mass Customization implementation, this paper introduces return policy into MC supply chain with upstream modular maker and downstream assembler, and establishes the centralized and decentralized decision-making models under Electronic Commerce, then dicusses respectively the optimal return rate and modularity for modular maker and assembler, and impact on lead time and selling price. Some results show that the optimal return policy not only rely on the degree of modularity, but also on demand volumn and the choice of decision-making in supply chain. In addition, there is the problem of ‘double marginalization’ under decentralized decision-making under EC, so we coordinate supply chain wih Nash bargining model contract. Finally, we further verfiy the conclusion by using numerical simulations, and give a number of suggestions and insights.