Embodiments of the present invention may provide an apparatus and a method for displaying a 3-dimensional ultrasound image formed based on 2-dimensional ultrasound images. The method for displaying an ultrasound image, comprises: a) forming and storing a plurality of sequential 2-dimensional ultrasound images based on ultrasound echo signals reflected from a target object, each of said sequential 2-dimensional ultrasound images being assigned a serial number; b) selecting N numbers of 2-dimensional ultrasound images having consecutive serial numbers; c) superposing the N number of 2-dimensional ultrasound images to form a 3-dimensional ultrasound image; d) forming a flow direction marker indicating a first 2-dimensional ultrasound image from the N numbers of 2-dimensional ultrasound images; e) displaying the 3-dimensional ultrasound image together with the flow direction marker on a screen; f) removing the first 2-dimensional ultrasound image from the N numbers of 2-dimensional ultrasound images; g) selecting a (N+1) th 2-dimensional ultrasound image and superposing the selected 2-dimensional ultrasound image to the superposed 2-dimensional ultrasound images; and h) repeating the steps c) to h) as many as a predetermined number.