The roll draft mechanism effects not only the quality of the yarn, but also the manufacturing efficiency in the yarn formation processes. The regularities of slivers and fiber bundles can improve the continuing process by doubling and drawing, where the roll draft mechanism plays the most important role. In this research the dynamic behavior of the output sliver thickness in roll drafting was modeled and the model parameters verified experimentally. As the results of the research, it reveals that 1) the dynamic thickness change shows different behaviour according to the type of the input variables(i.e. ; input sliver thickness, draft ratio), 2) the dynamic model has the 8th order with transportation lag time (395 $ imes$5.3msec) when a strand of the input slivers is broken down and 3) the variation of the draft ratio effects the thickness of the output sliver in the form of the 3rd order system with the transportation lag time (955 $ imes$5.3msec) in the circumstances of the delivery speed of the output sliver, 25m/min.