The deformation behaviors of three types of polyethylene (PE) with different molecular weights and short chain branch contents were studied by in-situ Fourier transformation infrared microspectroscopic imaging (FTIRI) with a Focal Plane Array (FPA) detector during uniaxial tensile test. The crystal orientation distributions within a 250 x 250 mu m(2) region during tensile test were obtained, especially in the front of necking profile. The results show that either increasing the molecular weight or adding the short chain branches could enhance the resistance of crystal to be orientated. With the aid of the Landau-de Gennes theory of nematic-isotropic transition, the spatial distribution of crystal orientation during the steady neck propagation is quantitatively analyzed, coupling with its corresponding mechanical behavior coherently. The theoretical analysis reveals that the constant Phi(0) and the coefficient of the Gaussian term A in the Landau-de Gennes model are valid parameters to evaluate the mechanical property of PE materials, which may be generalized as a new method to quantify the mechanical property of semi-crystalline polymers. (C) 2012 Elsevier Ltd. All rights reserved.