Abstract: After a discussion of the communication overhead and the principles of thread scheduling in software DSM, this paper presents the scheduling model based on TIG（thread interaction graph）. A heuristic thread scheduling algorithm is then presented, which assigns threads to processors in multithreaded software DSM. The algorithm is evaluated using 40,000 random generated TIGs, and implemented in a software DSM, named SMONN（shared memory on network of NT workstations）. Performance data of evaluation and implementation are also given.