Existing research demonstrates that an effective routing and wavelength assignment (RWA) strategy and a proper wavelength converter placement algorithm are the two primary vehicles for improving the blocking performance of wavelength-routed network. However, these two issues have largely been investigated, separately in that the existing RWA algorithms seldom consider the presence of wavelength conversion. We argue in this paper that any RWA algorithm needs to take into account the underlying wavelength conversion for two reasons: (1) wavelength converter placement is usually done at a much earlier stage during capacity planning; and (2) one of the key advantages of an all-optical network is its reconfigurability in that the network topology can be changed through routing and wavelength assignment. We propose a weighted least-congestion routing and first-fit wavelength assignment (WLCR-FF) RWA algorithm in conjunction with a heuristic wavelength converter placement algorithm called minimum blocking probability first (MBPF) that considers both the distribution of free wavelengths and the lengths of each route jointly. Using both analysis and simulation, we carry out extensive studies to compare the performance of the proposed algorithms over a variety of topologies. The results demonstrate that the proposed WLCR-FF algorithm can achieve much better blocking performance than static routing, fixed-alternate routing and least-loaded routing algorithms, in the environment of sparse or/and full wavelength conversion.