1. An ultrasound system, comprising: an ultrasound data acquisition unit configured to transmit and receive ultrasound signals to and from a target object to acquire ultrasound data; a volume data forming unit configured to form volume data by using the ultrasound data; an user input unit for allowing a user to input a user instruction; and a processing unit configured to set a plurality of volume slice regions having different widths in the volume data in response to the user instruction and form a plurality of 3-dimensional ultrasound images by using volume slices defined by the volume slice regions.