This paper presents a new level of description between behavioral and state descriptions of a finite-state machine (FSM). The description is termed behavioral phase clustering description. New concepts of behavioral phase and clustering of behavioral phases in an FSM are introduced. The new description simplifies functional analysis, verification and test of FSM designs. If an FSM is described at low level, some states can be clustered into behavioral phases directly. If it is described at behavioral level, behavioral phases can be extracted from the behavioral description, and clustering of behavioral phases can be performed through easy functional analysis. As one application of behavioral phase clustering descriptions, a new technique employed in a test generation system, ATCLUB, at Register Transfer (RT)-level based on a behavioral phase transition fault model is introduced in this paper. In ATCLUB, test generation process is accelerated through clustering of behavioral phases. Experimental results This paper presents a new level of description between behavioral and state descriptions of a finite-state machine (FSM). The description is termed behavioral phase clustering description. New concepts of behavioral phase and clustering of behavioral phases in an FSM are presented. The new description simplifies functional analysis, verification and test of FSM designs. If an FSM is described at low level, some states can be clustered into behavioral levels directly, behavioral stages can be extracted from the behavioral description, and clustering of behavioral phases can be performed through easy functional analysis. As one application of behavioral phase clustering descriptions, a new technique employed in a test generation system, ATCLUB, at Register Transfer (RT) -level based on a behavioral phase transition fault model is introduced in this paper. In ATCLUB, test generation process is accelerated through clustering of behavioral phases. Exp erimental results