1. An ultrasound system, comprising: a user input unit configured to receive input information for setting at least one seed point on a two-dimensional ultrasound image from a user; an ultrasound data acquisition unit configured to transmit and receive ultrasound signals to and from a target object including a plurality of objects of interest and output ultrasound data; and a processing unit in communication with the user input unit and the ultrasound data acquisition unit, the processing unit being configured to form volume data based on the ultrasound data, form the two-dimensional ultrasound image and a three-dimensional ultrasound image based on volume data, detect at least one object of interest corresponding to the input information from the three-dimensional ultrasound image, and perform an image process and a size measurement of the detected object of interest upon the three-dimensional ultrasound image.