Voids in a slab need to be closed by plastic deformation during rolling or forging processes. However, the progress of the void closure differs for the location of a void since the stress distribution in the slab is inhomogeneous. In the present investigation, the voids were assumed to be cylindrical and aligned in the thickness direction, and the void closure at any location was attempted to be predicted during the plane-strain compression in the width direction. A series of finite element analyses were performed, and as a result relations among principal strains, minor axis and aspect ratio of the cross section of a void were obtained with which the prediction of the void closure became possible. Applications of these relations to forging and rolling processes were successfully performed.