Embodiments for providing a three-dimensional (3D) ultrasound image 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 output a plurality of ultrasound data a user input unit configured to receive first input information for defining a region of interest (ROI) from a user and a processing unit in communication with the ultrasound data acquisition unit and the user input unit, the processing unit being configured to form volume data based on the plurality of ultrasound data, define the ROI in the volume data in response to the first input information, and render volume data corresponding to the ROI to form a three-dimensional (3D) ultrasound image, wherein the user input unit is further configured to receive second input information for defining a sub ROI on the 3D ultrasound image, and wherein the processing unit is further configured to define the sub ROI in the volume data based on the second input information, and render volume data corresponding to the sub ROI to form a sub 3D ultrasound image.
