1. An ultrasound system, comprising: an ultrasound image forming unit including an ultrasound probe and being configured to form a three-dimensional ultrasound image of a target object; a sensor coupled to the ultrasound probe; a memory configured to store a three-dimensional computed tomography (CT) image of the target object and position information on a position between the three-dimensional ultrasound image and the sensor; and a processor configured to extract a same anatomic region from the three-dimensional ultrasound image and the three-dimensional CT image, perform image registration between the three-dimensional CT image and the three-dimensional ultrasound image based on the same anatomic region to thereby form a first transformation function for transforming a position on the three-dimensional CT image to a corresponding position on the three-dimensional ultrasound image, form a second transformation function between the three-dimensional ultrasound image and the sensor based on the position information and perform calibration of the sensor to match coordinates of the CT image and coordinates of the sensor based on the first transformation function and the second transformation function.