1. An ultrasound system, comprising: an ultrasound data acquisition unit configured to sequentially acquire ultrasound data corresponding to a living body; and a processing unit configured to sequentially form a plurality of ultrasound images based on the ultrasound data, estimate a frame average coefficient corresponding to a change of a frame rate based on an i th (i is a positive integer) ultrasound image and an i th frame average image, the processing unit being further configured to perform a frame average process between the i th frame average image and an (i+1) th ultrasound image based on the frame average coefficient to form an (i+1) th frame average image.