Abstract: In this paper, starting from Hart's constitutive equation, introducing initial imperfection in general sense and considering early theories of S-H, we put forward a theorecal andlysis on plastic deformation and necking development under both uniaxial stress and biaxial stress. An unified integral expression suitable for both uniaxial and biaxial stresses is first given, in which Hutchinson et. Al's expression (1977) con be included as a special case. Based on it, a simple analytic expression is first given as follows.