A method of generating a diagnostic image of a subject including transmitting a transmission signal to the subject; acquiring a first RF frame and a second RF frame from an echo signal reflected from the subject; estimating a second directional displacement indicating a degree of movement of a first point of the subject appearing in the first RF frame in the second RF frame; generating an ultrasonic image corresponding to the second RF frame; and correcting an error of the generated ultrasonic image using the estimated second directional displacement. The first point may appear in the first RF frame and the second RF frame.