This paper presents a string matching hardware on FPGA for network intrusion detection systems. The proposed architecture, consisting of packet classifiers and strings matching verifiers, achieves superb throughput by using several mechanisms. First, based on incoming packet contents, the packet classifiers can dramatically reduce the number of strings to be matched for each packet and, accordingly, feed the packet to a proper verifier to conduct matching. Second, a novel multi-threading finite state machine (FSM) is proposed, which improves FSM clock frequency and allows multiple packets to be examined by a single FSM simultaneously. Design techniques for high-speed interconnect and interface circuits are also presented. Experimental results are presented to explore the trade-offs between system performance, strings partition granularity and hardware resource cost