The study deals with the incremental stretch-expanding process of a sheet metal using hammering. Although current incremental forming is a method of plastic working that employs basically static force, it is also possible to use dynamic force through impact, such as through free forging and hammering. If dynamic force is used, more flexible thickness control is expected compared with static one. The study has constructed the work method and succeeded to form some comparatively primitive shapes. However, formability of the work results differs according to the tool paths in this method. Thus, unsuitable tool paths threaten to cause both deterioration of work accuracy and extreme thickness decreasing or fracturing. However, relationship between the tool paths and formability has not been discussed sufficiently. Therefore, in this study, tool paths which the work method can select is classified correctly, and effect of each tool path on strain distribution is particularly experimented and discussed to clarify the forming principle. Furthermore, an optimum tool path generation method is discussed based on these conclusions. As the result, flexibility of thickness control as an advantage of the method is confirmed.