Abstract: Embodiments for forming an elastic image in an ultrasound system are disclosed. In one embodiment, an ultrasound data acquisition unit is configured to transmit/receive first ultrasound signals to/from a target object to output first ultrasound data and to transmit/receive second ultrasound signals for applying an acoustic radiation force impulse (ARFI) to/from the target object to output second ultrasound data. An image forming unit is configured to form a reference image based on the first ultrasound data and to form an ARFI image based on the second ultrasound data. A processing unit is connected to the ultrasound data acquisition unit and the image forming unit. The processing unit is configured to detect first feature points of the target object in the reference image, to detect second feature points of the target object in the ARFI image and to calculate stress data at each of the first feature points based on the first and second feature points.