Optimal BGP route selection on traffic demand is one of the problems in interdomain traffic engineering. Determining bottleneck area will give important heuristic information to the problem. As the problem of determining bottleneck area is NP-hard, a bottleneck area predicting algorithm on traffic demand in polynomial time is proposed, which deals with interdomain peering links and intradomian links simultaneously. Moreover, this paper also analysis the relationships between the traffic and the bottleneck area, as well as the relationships between the topology and the bottleneck area. Simulation results show the accuracy of the algorithm is more than 90%. In addition, a conclusion is drawn from the simulation that the topology is a very important factor in determining the bottleneck area.