The crystal orientations of linear polyethylene films on silicon substrates are investigated using grazing incidence X-ray diffraction and atomic force microscopy. From diffraction analysis, we can identify the structural arrangement of PE crystals in ultrathin film. The orientation of lamellar crystal in PE films changes from edge-on to flat-on with the decrease of film thickness in the film thickness below $ 100 nm. The slightly inclined lamellae relative to the substrate are found to coexist with the flat-on lamellae in thin PE films that we have investigated. We find that the crystal orientation and structures is governed by the constraint imposed by film thickness rather than enthalpy gain as the film got thinner especially in the thickness below 200 nm.

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