A successful completion of a project entails the proper use of project management. Project management includes different areas of which scope, as a major one, requires identification of the work that should be completed in a project. This is greatly facilitated by the use Work Breakdown Structure (WBS), of which Work Packages (WPs) are the basic components. This study aimed at identifying major criteria used by project managers, to establish the work content of the WPs included in the project. The study is based on analysis of 25 projects by their project managers. As part of the analysis, project managers requested to analyze a sample of WPs from each project generating a total of 219 WPs that were analyzed. Five major groups of WPs nature were identified, namely' design, testing, management, production, and procurement. Project managers were asked to identify the rule used by them to stop further decomposition of WPs. Six major rules were identified, which captured 93% of the rules used by project managers in establishing the WP size.