Institutional theory has gained special interest in management accounting research. Some research in management accounting literature tries to depict the change in management accounting systems using elements both from institutional and contingency theories. However, some other research tries to examine the integral impact of institutional and contingency factors on the use and design of costing systems. This research aims at exploring the complementary impact of both institutional factors, informed by new institutional sociology, and other inter- organization contingency factors on the use and design of costing systems in the General Establishment of Drinking Water in Tartous. A longitudinal case study is used in this research, and qualitative data are collected using personal interviews, observations, and examining company documents. The results of this research show that the use and design of costing systems in the case being investigated are particularly influenced by institutional pressures and inter-organizational factors, top management support, the availability of trained staff, and the co-operation of other departments.