Embodiments for forming a color map necessary for forming an elastic image in an ultrasound system. A transmission/reception (Tx/Rx) unit transmits ultrasound signals to a target object and receives ultrasound echo signals reflected from the target object to thereby obtain receive signals. The receive signals include first receive signals obtained before applying a stress to the target object and second receive signals obtained after applying a stress to the target object. An ultrasound data forming unit forms first and second ultrasound data based on the first and second receive signals, respectively. A processing unit computes strains by using the first and second ultrasound data and form a histogram based on the computed strains. The processing unit further clusters the histogram for separation into a plurality of clusters and forms a color map including a plurality of color map regions corresponding to the respective clusters.
