Abstract: Clustering trajectories as a whole can miss common sub-trajectories.Discovering common sub-trajectories is very useful in many applications，such as weather forecast and traffic control.A new method T-CLUS is proposed to clustering trajectories，which partitions a trajectory into line segments，and then the augmented cluster-ordering of the line segments is generated，finally cluster structure of sub-trajectories is identified by means of reachability plot.Experimental results demonstrate that T-CLUS is scalable and accurate to discover common sub-trajectories from a trajectory database.