Clustering data mining divides data objects into several groups or clusters automatically so that the objects in the same group have high similarities, and the objects in different groups have big differences. Clustering algorithms have been studied for many years. The typical algorithms are k-means and k-medoids In k-means, every group is represented by the average value of the objects in the group. The clustering process is: randomly choose k objects, each object initially represents the average of each group, assign each remaining object to one of the k groups to which the object has the shortest distance. CalcuIate the average of each group, and regroup all objects according to the distance to the new averages. Do average and regroup steps again until the groups are no longer changed. In k-medoids, each group is represented by the object that is at the center of the group. The clustering process is similar to k-means. Above algorithms need scan instance data many times. Microsoft@ provided an efficient algorithm - Scdable Expectation Maximization (SEM) [21(31, which only scans data one time. The basic idea is to create clusters by the density of the instance objects. The calculation process can be stopped anywhere and restarted again. We can get a reasonable result at any point in the process. The algorithm creates some clusters when processing data records, and changes the centers of the clusters while more data are processed in order to find a duster set that can describe the characteristics of similar instance objects best.