Two 3D finite element models of the Ultrasonic Impact Treatment (UIT) process, a single-impact model and a two-impact model, are presented to simulate pin impact on 2024 aluminum alloy. In the singleimpact model, the transformation of the energy and transmission of the stress waves are used to analyze the distributions of the stress field and equivalent plastic strain field during the contact and rebound processes. Further, the effects of the initial impact velocity, pin size, and shape on the residual stress distributions are investigated. In the two-impact model, the two-impact process with different distances is investigated to analyze the effects of the second impact on the residual stress field induced by the first impact. (C) 2015 Elsevier BAT. All rights reserved.