Abstract: With the development at top speed of VLSI technology, especially the appearance of RISC and ASIC, behavior funclion level simulation has becomc a necessary tool of digital system designers.This paper presents a powerful and pract ical CAD tool for designers-behavior function level simulation system (BFLSIM). In this paper, we systematically discuss the function, the structure, the characte ristics and simulation model of BFLSIM. In the end, we introduce a flexible and convenient user's interface-DCL and DEBUG environments, which can increase the system efficience and practicality.