Determination of the primary shear deformation zone (PSDZ) to evaluate tool-chip interface temperature and forces during metal cutting processes is important. Evaluation of the PSDZ can be performed either experimentally or analytically. Experimental methods are cumbersome and time-consuming. Hence, an analytical method is proposed. Metal undergoes a large plastic deformation during cutting and behaves like an incompressible, non-Newtonian fluid. The material behaviour, therefore, is modelled by a viscoplastic constitutive equation. Temperature effects are included by evaluating the material properties at a typical average temperature usually encountered in the cutting zone. Only orthogonal steady state machining is considered so that the problem of metal flow in the cutting zone can be modelled as a two-dimensional, steady state problem. The finite element method (FEM) is used to convert the continuity and momentum equations to a set of non-linear algebraic equations that are solved by the frontal method. The results obtained for the average shear strain rate, shear flow stress and mean width of the PSDZ compare favourably well with the experimental results. The PSDZ is found to depend basically on feed rate and cutting speed for a given workpiece material, which is consistent with the experimental observations of others. It is concluded that a 3% cut-off for the maximum strain invariant can be used to determine the mean width of the PSDZ.