A method and a system of automatically detecting a boundary of a target object by using an ultrasound diagnostic image. In accordance with the present invention, the boundary of the target object can be detected automatically and precisely. The ultrasound diagnostic method and system form an edge detection boundary candidate model by detecting an edge of the ultrasound diagnostic image, forming a simplification boundary candidate model by performing a simplification operation to the edge detection boundary candidate model, forming a sessionization boundary candidate model by performing a sessionization operation to the simplification boundary candidate model, and detecting the boundary of the target object of the ultrasound diagnostic image based on the edge detection boundary candidate model, the simplification boundary candidate model and the sessionization boundary candidate model. In accordance with the present invention, the boundary of the target object is automatically detected by using the ultrasound diagnostic image to reduce any inconvenience to the user and solve the problem in which the measured result may vary with each user, which was caused by a conventional method of manually detecting the boundary.