Application of object-oriented design patterns for the development of power system computation software is presented. Important patterns such as "Iterator," "Decorator," and "Template Method " are utilized to develop a comprehensive power flow (load flow) analysis program, including FACTS devices such as SVC, TCSC, STATCOM, and UPFC. Analysis-independent object models of power system components are obtained by untying the attributes required for computation from the attributes representing the physical characteristics of an object. The proposed object-oriented design of power flow computation has been implemented in C++ on windows environment. The performance of the developed program has been tested with IEEE benchmark systems. © 2006 IEEE.