The present study aims at looking into the nature of the horseshoe vortex system and its interaction with the wake-flow region in the base of circular cylinder mounted on a base plate in deep water. These kinds of flows have been attracting considerable attention due to its importance in engineering applications. Flow fields in plan and side-view planes were visualized using a laser based particle image velocimetry to examine the flow structure quantitatively for the range of Reynolds number 1500 <= Re-d <= 9600. The analysis of the cinema of instantaneous flow data, time-averaged velocity vectors map, streamlines, vorticity patterns, Reynolds stress correlations and fluctuations of the velocity components clearly demonstrate that the flow structure occurred in the base of the cylinder is three dimensional. The area of flow field occupied and affected by the horseshoe vortex system around the cylinder varies as a function of Reynolds number. This horseshoe vortex system emanating from the upstream of the cylinder swirls while travelling in the main flow direction. Counter-clockwise rotating vortices take place downstream of the cylinder in vertical plan-view plane. These swirling and rotating vortices in the junction region of the plate and cylinder cause scour mechanisms. Horseshoe vortices magnify the entrainment process and circulatory motions between the main and wake-flow regions downstream of the cylinder which stimulate the scour mechanisms. (C) 2008 Elsevier Ltd. All rights reserved.