Lightweight composite sandwich structures are composed of composite structures that are laminated between thin stiff facesheets bonded to a thicker lightweight core. These structures have high potatial to be used in civil engineering applications, marine, aerospace industry etc. applications due to their high strength to weight ratios and energy absorption capacity. In these structures, the bending loads are generally carried by the force couple formed by the face sheets while the shear loads are carried by the lightweight core materials. Main purpose of the core material is to provide a high moment of inertia. Therefore, under flexural loading, sandwich panels have higher specific mechanical properties relative to the monocoque structures. Also, the core resists transverse forces and stabilizes the laminates against global buckling and local buckling. The resulting structure provides increased buckling resistance and its rigidity. In this study, sandwich composite structures were developed with carbon fiber reinforced polymer composite facesheets and the cores made by Aluminum (Al) based honeycomb with various thicknesses. Carbon fiber/epoxy composite facesheets were fabricated with non-woven unidirectional (UD) fabrics (with 0o/90o orientation) and epoxy resin by vacuum infusion technique. Al honeycomb layers were sandwiched together with carbon/epoxy facesheets using a thermosetting adhesive. Mechanical tests were carried out to determine the mechanical behavior of face sheets, aluminum cores and the composite sandwich structures. Effect of core thickness on the mechanical properties of the sandwich structures was investigated. Keywords: Sandwich structures, carbon fiber, epoxy, aluminium honeycomb, mechanical properties, vacuum infusion.