The active noise control (ANC) technique is used to cancel original noise by actively generating a secondary sound signal. Actually it is a method using the interference effect of sound waves. The first part of this paper will introduce and discuss the ANC technique, including the basic concepts and principles, history, current development, and application. Some application examples of ANC technique in duct noise control are given here. The second part will introduce the necessary digital signal processing knowledge and system identification technique which is used in this noise control system. The mathematic model of the sound feedback channel and error signal channel is established by using the system identification technique. The third part of this paper will introduce the ANC experiment results in an air-duct, and an important conclusion is given. Many factors related to ANC are investigated; these factors include the position of the microphones in the duct, the vibration of the shells of the duct, the reflection of the sound waves from the end of the duct, ...<th>. Very excellent results were achieved by using this system; it reduces the wide-band noise in the range of 0–800 Hz effectively; 30-dB reduction of sound power level was obtained to the white noise of 250 Hz oct.