In contour grinding process with ball-end grinding wheel, successive cutting point spacing and also wheel peripheral speed change continuously on a working wheel surface. And these parameters will have an influence on grinding mechanism. Then, in order to make grinding mechanism clear, undeformed chip shape is theoretically analyzed in this paper. Fundamental relationships about substantial parameters, which represent the undeformed chip shape, such as chip thickness, chip length, interference angle and cross-sectional chip area, are theoretically analyzed. Then, in order to obtain these parameters, numerical simulation method is established. The undeformed chip shape is symmetrical form to rotational angle of cutting edge, and is extremely longer and also thinner than that in ordinary external cylindrical plunge grinding. Effects of tool and grinding conditions on the undeformed chip shape are investigated, and it made clear that an increase of wheel peripheral surface speed should be reduce cutting force affected on a grain. Furthermore, grinding force distribution on a wheel working surface is predicted considering with the chip thickness and the wheel peripheral speed.