Network flow approaches have been used for partitioning with success in the past. However, most of them can not deal with size constraints directly in partitioning. Instead, they incorporate the size constraints implicitly in the objective function. This paper presents a new network flow approach for partitioning circuits into tree hierarchies. We formulate a linear program for the hierarchical tree partitioning problem by spreading metrics proposed in [3][4]. The size constraints in partitioning can be formulated directly as linear constraints. Motivated by the duality between the linear programs for partitioning and network flow problems, we devise a heuristic algorithm based on network flow and spreading metric computations. Experimental results demonstrate that the new algorithm can generate better solutions for MCNC benchmarks.