1. An ultrasound system for forming an ultrasound image by using a plurality of sub-frames, comprising: a curved linear probe including a transducer array, the transducer array comprising a plurality of transducers configured to transmit and receive ultrasound signals along a plurality of scan lines to obtain sub-frames; a determination unit configured to set a common point where extensions of the scan lines with respect to the transducers intersect at a back side of the transducer array and move the common point in a plurality of predetermined directions to determine a plurality of virtual common points at the back side of the transducer array, the determination unit further being configured to determine a plurality of transducer groups, steering angles, and a plurality of scan-line groups, wherein the plurality of scan-line groups correspond to the plurality of transducer groups and the plurality of virtual common points, respectively; a controller configured to control the transmission of the ultrasound signal along the scan lines of the scan-line groups corresponding to the respective sub-frames based on the plurality of transducer groups and the steering angles; and a processor configured to synthesize the plurality of sub-frames to form an ultrasound image.