A new algorithm is proposed using the feed-forward type shared FDL bank to achieve service differentiation in optical burst switching networks, which is composed of many fiber delay lines. In our scheme, the proposed algorithm divides a shared FDL bank into sub-groups, and the different sub-group reservation policies for each class are related to each other. By doing so, the algorithm can accomplish service differentiation easily. The optimal number of delay lines for each sub-group is decided by the algorithm. The scheme is verified through extensive simulations and numerical analysis.