We study the tolerance of complex networks to attacks. Due to the large network scale, it is almost impossible for an attacker to have the complete topology information about the whole network. Thus, an efficient attack strategy based on partial degree information is proposed. Using the generating function method we give the exact solution for the attack strategy. A theoretical scale-free random network and the real Internet data are considered as examples, and the results clearly show the performance degradation due to the lack of topology information. By comparing to previous strategies, the efficiency of our strategy is demonstrated.