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Soil-vehicle interaction is an important factor for predicting trafficability of off-road vehicles. Tractive forces on the wheel to drive the vehicle can be estimated by soil deformation analysis around the contact area of the wheel. Sand deformation and flow under a rolling wheel with slippage were observed from the side through a transparent acrylic wall by 2-dimensional Particle Image Velocimetry (PIV) method. The flow velocities of sand particles obtained on allocated grids revealed that periodic fluctuations of sand flow make ripple patterns on the sand surface behind the wheel. We discussed the effect of wheel slip ratios on the density of the ripples, sinkage, drawbar pull, torque and the depth of influence for the traveling wheel. The result showed that the slipping wheel forms a ripple with each scratch by the wheel rotation corresponding to the front half contact length with sand. The thickness of sand flow layer under the wheel showed thinner for large slip ratios. The finite element method was also conducted to simulate the traveling wheel experiment to discuss the possibility of the numerical simulation. The result of simulation qualitatively reproduced the experiment. However, there was a quantitative difference on the drawbar pull of the wheel, which shows that the method needs modification on the numerical model.