There are provided embodiments for providing a color Doppler image based on qualification curve information. In one embodiment, an ultrasound system comprises: an ultrasound data acquisition unit configured to acquire ultrasound data corresponding to a living body including a moving target object; a storage unit for storing qualification curve information for determining blood flow signals of the target object, clutter signals and noise based on velocity and power components of Doppler signals; and a processing unit configured to form first Doppler signals based on the ultrasound data, perform a clutter filtering process upon the first Doppler signals to form second Doppler signals, calculate velocity and power components of the second Doppler signals, form a color Doppler image based on the calculated velocity and power components, and perform a blending process upon the color Doppler image based on the calculated velocity and power components and the qualification curve information.