The characteristics of vortex rings induced below the free water surface due to the impingement of a falling single drop with small drop diameters and two impact angles were investigated by means of a three-dimensional direct numerical simulation (DNS) with a Level-Set method. The results show that the present DNS well captures the structures and transport processes of a crater vortex-ring (CV) and a water-column vortex-ring (WCV) due to the vertical impingement of a drop, and that the intensity of the WCV is larger than that of the CV. In the oblique impingement of a drop, a CV is similarly generated, but a vortex tube (VT) creeping along the free surface is also generated instead of the WCV, and the intensity of the VT is much smaller than that of the CV. The relationship between the momentum of a drop and the intensity of the dominant vortex ring is determined independently of diameter and impact angle and is also consistent with the previous measurements (Takagaki and Komori, 2014). (C) 2014 Elsevier Ltd. All rights reserved.