In the mixed-line-rate (MLR) wavelength-division multiplexing (WDM) networks, for each wavelength of fiber, it can provide different bandwidths in 10/40/100 Gbps by using different modulation formats. In this paper, the virtual topology design (VTD) problem is studied for allocating the multi-hour traffic on MLR-WDM networks. The goal is to construct a feasible virtual topology to meet the traffic requirement such that the total cost of transponders can be minimized. A heuristic algorithm is proposed to solve this problem and the simulating results are shown.