In this paper, we study the following problem. Given the network topology and traffic demand, determine how a minimum set of wavelength converters should be placed to ensure that the number of wavelengths needed will not exceed a given bound L+u, where L is the maximum link load in the network and u is a parameter defined by the network designer to reflect the overall availability of wavelength resources. This problem, however, is proved to be NP-hard. Hence we develop an efficient heuristic algorithm and extensive theoretical and experimental studies are carried out to verify the effectiveness and performance of the algorithm.