1. A method of acquiring an ultrasound image in an ultrasound imaging system having an array transducer, comprising: a) setting an image point at a predetermined depth on an ultrasound image acquired based on a reference speed of sound for a target object; b) setting a range of speed of sound with reference to the reference speed; c) setting a plurality of speeds of sound in a predetermined interval within the range of speed of sound; d) acquiring ultrasound images at each speed of sound; e) calculating an amplitude of an image point set at a predetermined depth of each ultrasound image; f) determining a real speed of sound in the target object based on the calculated amplitude; and g) acquiring an ultrasound image based on the determined real speed of sound in the target object.