SummaryOne of the important functions of a communication network is the routing. The communication and performance of a network whose efficiency can be directly affected by edge‐forwarding index of the routing and the edge‐forwarding index can be used to deterministically measure its efficiency. This paper investigates edge‐forwarding index of routing in the lexicographic product network, which is used to measure the congestion of an edge. Congestion of any edge is limited by the capacity of the edge; the original study of edge‐forwarding index was motivated by the problem of maximizing network capacity, which is achieved by minimizing the forwarding index of a network. Thus, it is an important parameter to measure the quality of a routing. By applying the combination design and interconnection network, we characterize a closed upper bound on edge‐forwarding index of the lexicographic product network. Such a boundary depends only on the parameters of the factor network, such as the edge‐forwarding index of the factor network.