The prediction of roll motion of a ship with bilge keels is particularly difficult because of the nonlinear characteristics of the viscous roll damping. Flow separation and vortex shedding caused by bilge keels significantly affect the roll damping and hence the magnitude of the roll response. To predict the ship motion, the Slender-Ship Free-Surface Random Vortex Method (SSFSRVM) was employed. It is a fast discrete-vortex free-surface viscous- flow solver developed to run on a standard desktop computer. It features a quasi-three dimensional formulation that allows the decomposition of the three-dimensional ship-hull problem into a series of two-dimensional computational planes, in which the two-dimensional free-surface Navier-Stokes solver FSRVM can be applied. In this paper, the effectiveness of SSFSRVM modeling is examined by comparing the time histories of free roll- decay motion resulting from simulations and from experimental measurements. Further, the detailed two-dimensional vorticity distribution near a bilge keel obtained from the numerical model will also be compared with the existing experimental DPIV images. Next, we will report, based on the time-domain simulation of the coupled hull and fluid motion, how the roll decay coefficients and the flow field are altered by the span of the bilge keels. Plots of vorticity contour and vorticity iso-surface along the three-dimensional hull will be presented to reveal the motion of fluid particles and vortex filaments near the keels.