The interfacial structure of laminated poly-alpha-olefin, polyethylene (PE)/polypropylene (PP), film was investigated by scanning thermal microscope (SThM). Spatial resolution of this technique was estimated as 1.5 mu m. Notwithstanding the flatness of the sample as observed in the height image of an atomic force micrograph, micro-thermomechanical analysis could distinguish each polymer. By scanning the thermal probe at isothermal temperature over a cross-section of the laminate film, a high-contrast image of apparent thermal conductivity at the interfacial region was obtained. The interfacial thickness was estimated to be in the micrometer order depending on the thermal treatment of the laminate film. This result coincides with the results of synchrotron X-ray microbeam diffraction. Moreover, the peel strength of the PE/PP laminate was considered to be influenced by the interfacial thickness. It is concluded that this proposed thermal-probe method is as a powerful tool for investigating the interfacial region of the polymer composites. (C) 2011 Elsevier B.V. All rights reserved.