This paper proposes a performance estimation method for multithreaded processor systems. Multithreaded processors are known as effective memory latency tolerating scheme with multiple hardware contexts in a processor. It is important to estimate the processor working ratio for comparing various configurations. The proposed method is based on, firstly, assuming independent cache miss occurrences, and modeling the processor status with number of runnable threads and status of the thread in the execution stage, then employing a probability transition matrix between all possible processor statuses. Our method drastically reduced the amount of computation required to estimate the working ratio than traditional simulations.