This paper deals with two methods of analyzing and planning an assembly work. First, we propose a method of breaking the assembly work down into its work elements, in which all parts constituting the product are classified into unit groups based on the fastener method in assembling (e.g. bolted joints, riveted joints, shrinking and pressing fits, and so on). As a result, the assembly work is broken down into its work elements which fasten the parts in each group. At the same time, a problem of precedence relation among them arises. Secondary, we propose an assignment method in which the number of stations is determined, and the work elements are assigned to the work stations in a single assembly line as the efficiency of line balancing alters as a variable. By using the methods proposed here, we could analyze the assembly work consistently through breaking it down into work elements and assigning the work elements to work stations.