Due to the nature of wireless transmission, communication in wireless mesh networks (WMNs) is vulnerable to many adversarial activities including eavesdropping. Pairwise key establishment is one of the fundamental issues in securing WMNs. This paper presents a new matrix based pairwise key establishment scheme. Mesh client in our scheme only needs to prestore a key seed, which can be used to generate a column of secret matrix. It can establish pairwise keys with other clients after mesh routers broadcast public matrices. Our scheme is motivated by the fact that in WMNs, mesh routers are more powerful than mesh clients, both in computation and communication. Besides, we employ the pre deployment knowledge to reduce the computational cost of mesh clients. Security and complexity analysis show that the new scheme possesses several desirable features: 1) neighbor mesh clients can directly establish pairwise keys; 2) the new scheme is updatable, scalable, and robust against node capture attacks; and 3) communication and storage costs at mesh clients are significantly reduced.