Conditions governing the roughening phenomenon of free surface in deformation processes are investigated. It is shown that the surface roughness increases with strain, and the roughening rate is affected by the mode of deformation (tension and compression), and by the position of the surface. The effects of the deformation mode and the position of the surface are related with change of the surface area during deformation. It is also found that the roughening rate is affected by grain size and the lattice structure of the metal, i.e. the roughening is remarkable in the metal of large grain size and of small number of slip systems. Wave length of the roughness is about 10 times as large as the grain size. Theoretical analysis of the wave length is carried out on the basis of probability.