Embodiments of adaptively performing clutter filtering are disclosed. In one embodiment, by way of non-limiting example, an ultrasound system comprises: an ultrasound data acquisition unit configured to transmit and receive ultrasound signals to and from a target object to thereby output a plurality of ultrasound data for obtaining a color Doppler mode image, wherein the target object includes at least one of a tissue and a blood flow; and a processing unit placed in communication with the ultrasound data acquisition unit and being configured to locate the plurality of ultrasound data on a complex plane, the processing unit being further configured to perform a circle fitting upon the plurality of ultrasound data located on the complex plane and perform a downmixing and a clutter filtering upon the circle-fitted ultrasound data in consideration of speed of the tissue.