Channel assignment in multi-channel multi-radio wireless mesh networks is a powerful management tool to exploit available resources efficiently. In this paper, we study the problem of dynamic channel assignment in presence of traffic with QoS constraints, to optimize network performance which is measured in terms of demands acceptance rate. We propose an on-line on-demand dynamic algorithm for the problem. It reassigns channels only when a demand cannot be accepted using current channel assignment and keeps the number of channel reassignments small by changing only the channels of the links in a vicinity of routing path of each demand. Comparisons with others algorithms, including the optimal static channel assignment, show that the propose...