Abstract: In this paper a new exact algorithm is proposed for solving project scheduling problems under multiple resource constrains which is based on the branch and bound procedure. The enumeration tree is generated by the feasible branches emanating from a parent node subject to precedence and resource constraints in the network. Then the feasible branches are evaluated by using the pruning rules based on the breadth-first search solution strategy, and the worse ones will be eliminated from the enumeration tree while the better will continue branching until they get the best solution. It is proved by examples that this new exact algorithm is efficient, providing a method for resource balance and a reasonable trade-off between resource allocation and duration.