This paper discusses a method of modeling Globally Asynchronous Locally Synchronou (GALS) systems by the use of Stochastic Timed Petri Nets(STPN), and a performance evvaluation tool. The operation of hte system is classifie into three operations; the foundamental component operation, synchronous data communication and asynchronous communication. Every operation is modeled by the use of STPN and model of (GALS) systems can be given by unifying these three models. The upper bound of the value of a evaluation index si derived as a maximam value among times separation of two events on timed executions of the STPN. According to a experimental result using the tool developed, the upper bound was calculated in about 8 hours for a (GALS) system which has 2 CD and 4 components.
