This paper defines and determines a graph model of a computer system in a form applicable to system performance analysis. The power of this modeling technique with respect to comprehensibility, accuracy of representation and ease of validation and modification is demonstrated by application to modeling of the UT-2 operating system for the CDC 6000 series computer system. This multiprocessor-multi-programmed operating system with its high degree of parallelism provides an excellent test for the utility and range of application of graph models in performance evaluation. A programmed representation of the kernel monitor is used. All other system processes are represented in graph form and are input data to the simulator. A generally applicable technique for extracting graph representations of processes are represented in graph form and are input data to the simulator. A generally applicable technique for extracting graph representations of processes from event trace data is described and applied to the event trace generated by the UT-2 operating system. The technique is both complete and general and may be profitably applied for either partial or complete models of any type of complex computer system process where data or techniques for automated graph construction are available or can he applied.