In this study, the probability unit ability levels of the eleventh grade Turkish students were classified through cluster analysis. The study was carried out in a high school located in Trabzon, Turkey during the fall semester of the 2011-2012 academic years. A total of 84 eleventh grade students participated. Students were taught about permutation, combination, binomial expansion, and probability, which were the subtopics of probability unit, in an individualized mathematics learning environment called UZWEBMAT. After students completed the learning of each sub-topic, they were subjected to an exam about the relevant topic through UZWEBMAT-CAT. Students participated in 5 separate exams (i.e. one for each sub-topic and one end-of-unit test). Data were collected via system records made up of the ability levels of students concerning each subject. The ability levels obtained from each exam were analyzed through hierarchical clustering. According to the study results, the ability levels of students gathered in two main clusters in every test: medium ability level and advanced ability level.