Active noise control techniques have been studied in recent years as an available method of machinery noise reduction. Especially, there are many applications of them to ductlike structures where sound waved radiated from noise sources can be assumed to be traveling as plane waves. In most of these cases, the lengths by ducts were assumed to be infinitely long, and acoustic properties such as reflections and resonances inside the ducts have seldom been considered. In actual machines, however, the duct lengths ate often short because of the limit of machine dimensions; hence a basic study on active noise control for such short-length ducts is needed. This paper describes the modeling method of an active noise control system using a fixed FIR filter for short-length ducts. The results of simulation bclculations were compared with experimental data and good agreement was obtained.