The present invention is directed to an ultrasound system capable of providing a plurality of M-mode images corresponding to M-mode lines without moving a probe. A volume data forming and reconstructing unit of the present invention forms volume data based on the ultrasound echo signals, the volume data forming and reconstructing unit being configured to determine a beat period of the moving object and reconstruct the volume data based on the beat period. A processor forms at least one reference image based on the reconstructed volume data and one or more M-mode images corresponding to one or more M-mode lines set on the reference image. A display unit displays the reference image, the M-mode lines and the M-mode images.