1. An ultrasound image display method comprising: displaying an ultrasound image of an object; selecting at least one region of interest (ROI) in the ultrasound image based on a user input; converting image pixel information corresponding to the at least one ROI into height values, wherein the converting of the image pixel information into the height values comprises determining a sign of the height values based on movement direction information of a tissue or a bloodstream when the ultrasound image is a color Doppler image or a spectral Doppler image; and three-dimensionally displaying a partial ultrasound image corresponding to the at least one ROI by using the height values, wherein the three-dimensional displaying of the partial ultrasound image corresponding to the at least one ROI comprises displaying the three-dimensional (3D) partial ultrasound image overlappingly on the ultrasound image which is a two-dimensional (2D) form, and wherein the displaying of the 3D partial ultrasound image overlappingly on the ultrasound image which is the 2D form comprises applying a semitransparent effect or a transparent effect to other images in the ultrasound image which is the 2D form except a region in which the 3D partial ultrasound image is overlappingly displayed.