In monitoring anomalous network activities, intrusion detection systems tend to generate a large amount of alerts, which greatly increase the workload of post-detection analysis and decision-making. In this paper, we propose a correlation approach based on sequential pattern mining techniques to fuse related alerts for the Distributed Denial of Service (DDoS) attacks. By mining the alert sequences and iteratively consolidating the matching sequential alert patterns, our approach is able to greatly reduce the related alerts and identify their DDoS membership. The alert reduction and fusing mechanism allow us to concentrate on a higher level of abstraction and thus save much extra efforts spent on analyzing a big volume of trivial raw alerts. Experimental comparisons of our method with hidden Markov model (HMM), a powerful stochastic process for sequence analysis, show that our algorithm is slightly better than HMM in terms of DDoS alert sequence identification.