Disclosed are an ultrasound diagnostic apparatus and an operating method thereof, which quickly and accurately set a volume of interest (VOI). The ultrasound diagnostic apparatus includes a data acquiring unit that acquires three-dimensional (3D) ultrasound data about an object, a display unit that displays a plane image of at least one of a plurality of planes included in the object, based on the 3D ultrasound data, a user input unit that receives a user input which sets a first straight line included in the plane image, a VOI setting unit that sets, as a VOI, a volume between a first flat surface, which is parallel to the first straight line and is separated from the first straight line by a first distance, and a second flat surface which is parallel to the first straight line and is separated from the first straight line by a second distance； and an image generating unit that generates at least one selected from a 3D image and a two-dimensional (2D) plane image of the set VOI, based on the 3D ultrasound data. The display unit displays at least one selected from the 3D image and the 2D plane image of the VOI.
