Making concurrent decisions on assembly plans and supply chain configurations (SCCs) is a desirable strategy in today's competitive global market. Such concurrent decisions help reduce decision iterations for shortened lead time and lower the total cost for manufacturers and their supply chains. However, there lacks deep understanding of the interaction between assembly plans and SCCs, as well as methodologies for integrating them. In this paper, we consider an assembly planning problem that simultaneously decomposes an assembly product to modules and assigns the modules to suppliers. The joint decision is formulated as an optimization problem of finding the assembly decomposition and the corresponding supply chain network with the lowest total supply chain cost. We develop a coordinated decision-making method by integrating assembly AND/OR graphs with supply chain cost information. A dynamic programing (DP) based algorithm is developed to efficiently solve the combined optimization problem. Case studies demonstrate that the proposed method can find optimal solutions for the integrated decision-making problem efficiently and provide a tool to plan assemblies from the perspective of the total supply chain cost in the early stages of product development.