This study classifies U.S. federal agencies into five types, based on Lowi’s well-known policy scheme. It constructs and tests five hypotheses to identify any structural differences in formalization, span of control, headquarters ratio, and personnel mobility between five types of agencies. The five hypotheses were generally confirmed. One important contribution of this study is that it made the first attempt to do hypothesis testing on Lowi’s agency types. Another contribution of this research is in its sub-classification of Lowi’s fourth category, constituent agencies, into staff and foreign defense agencies. According to the data analyses, each type of these agencies had distinctive organizational structures compared with other types. This legitimizes the sub-classification of constituent agencies. Consequently, this study argues that any significant differences in organizational structures were contingent on the type of policy mission pursued by an agency. Thus, civil service reformers should carefully consider various structural elements that are contingent on the type of agency when implementing reforms.