Abstract: An ultrasonic color Doppler imaging system, and particularly a filtering method for removing a clutter signal from an ultrasonic image is provided. A modified infinite impulse response (IIR) filter to which a moving target indicator (MTI) principle is applied, can sufficiently suppress an undesired transient phenomenon shown in a conventional general IIR filter, and realize a steep characteristic curve which is the same as that of the conventional IIR filter, using a small order. Thus, a frame rate of the image system can be improved and a real-time processing is possible. However, first three signals output from such a filter do not represent a characteristic of a filter accurately. Thus, first three kinds of data which cause an inaccuracy to a filter characteristic are removed from a filter processing, to render an accuracy in realization of a modified IIR filter.