Mobile Robots, which explore craters on the Moon or the Mars, are required mobility to negotiate with sandy slopes on which slippage can easily occur. It is known that such slippage can be reduced by sifting the center of gravity on robots. In this paper, we quantitatively discuss the effect of shifting of the center of gravity on wheeled mobile robots on side slip. We conducted slope-traversing experiments using a wheeled mobile robot under various roll angles and slope angles. Based on the experiments, we found the relationship between robot's roll angle and side slip. Furthermore, we also found that the slip is able to be minimized not by just making the robot horizontal, but by inclining it toward uphill direction more than such attitude. It indicates that to minimize side slip over slopes we have to focus on the mechanics model of wheels and optimize their load distribution.