Efficient performance tuning of parallel programs is often hard. In this paper we describe an approach that uses a uni-processor execution of a multithreaded program as reference to simulate a multiprocessor execution. The speed-up is predicted, and the program behaviour is visualized as a graph, which can be used in the performance tuning process. The simulator considers scheduling as well as hardware parameters, e.g., the thread priority, no. of LWPs, and no. of CPUs. The visualization part shows the simulated execution in two graphs: one showing the threads’ behaviour over time and the other the amount of parallel-ism over time. In the first graph is it possible to relate an event in the graph to the code line causing the event. Validation using a Sun multiprocessor with eight processors and five scientific parallel applications shows that the speed-up predictions are within +/-6% of a real execution.