This paper introduces a new technique employed in a test generation system, ATCLUB, at RT-level based on clustering of circuit states. States or some sets of states in a low-level description are mapped to high level in terms of a particular variable in a behavioral description, and termed behavioral phases in this paper. Further clustering of behavioral phases is performed to represent the function of a circuit more explicitly and refinedly. Such a refined representation is then used in the test generation algorithm to simplify and speed up search process of test sequences. Experimental results demonstrate the computational efficiency of the clustering process and test pattern generation.