Assembly devices are important in manufacturing systems. To study their conceptual design methodologies, it is necessary to investigate their behavioral process characteristics. In this paper, the behavioral processes exhibited by some assembly devices were studied, from which we discovered the so-called “phenyl-loop behaviors” with some unique characteristics. Some specific rules were subsequently proposed to help identify these behaviors, and their usefulness for conceptual design was also discussed. A structure-behavior database was set up based on existing assembly devices and a design platform was constructed for assembly devices design.