The theory of rolling given in the present paper is based upon the following assumptions:(1) There is no lateral spreading.(2) The material used is imconpressible,. isotropic, and viscous. It also obeys von Mises-Hencky's condition of yielding.(3) The elastic deformation of rolls is neglected.(4) The contact region between the material and rolls is divided into three parts: namely the forward slipping region, sticking region, and backward slipping region. The conditions of continuity of velocity and pressure are used.The results obtained from this theory agree very well with the experimental results given by Siebel and Lueg in 1933.