During a rod extrusion process, the oxidation layer and the segregation layer at the billet surface are possibly drawn inside the billet and become one part of the product, which portion with surface-permeation has to be cut off and results in a low productivity of the extrusion process. In this paper, the mechanism of permeation of the oxidation and segregation layers at the billet surface is explored using a finite element analysis. The effects of various extrusion conditions, such as extrusion ratio, inclination angle, billet length, the thickness of oxidation layer, etc., on the length of the portion with surface permeation are discussed systematically. Optimal inclination angles for a free surface-permeation product under different extrusion ratios are found out. An empirical equation for the optimal inclination angles is also proposed. Finally, experiments of extrusion of aluminum rods are conducted to validate the analytical model proposed.