1. An ultrasound system, comprising: an ultrasound image forming unit including a 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 perform image registration between the three-dimensional CT image and the three-dimensional ultrasound image to thereby form a first transformation function for transforming a position of the sensor to a corresponding position on the three-dimensional CT image and perform calibration of the sensor by applying the position information to the first transformation function.