Epitaxial crystallization is utilized to control both molecular chain orientation and microdomain structure in a thin film of a semicrystalline triblock copolymer, composed of crystallizable polyethylene (PE) end blocks and an amorphous ethylene-alt-propylene (PEP) midblock where the microphase separation is driven by crystallization from a homogeneous melt, characterized by small-angle and wide-angle X-ray diffraction. Surface interaction due to a crystallographic matching of unit cells between the crystalline PE block and benzoic acid (BB) substrate induces high orientation of the crystalline PE block, resulting in a well-ordered parallel lamellar microphase-separated structure. The excellent orientation induced by the surface interaction is evidenced by the selected area electron diffraction (SAD) pattern and bright-field (phase contrast) and dark-field (diffraction contrast) transmission electron microscope (TEM) images of the block copolymer thin film. The data clearly show that the chain axis ((c) over cap) of PE is parallel to the normal ((n) over cap) of the microphase-separated lamellar surfaces.