The forming of thin-walled aluminium housing by impact backward extrusion process was conducted, and the influence of metal microstructure on its formability was investigated. Aluminium alloys of A3003, AC4CH-F and semi-solid state of AC4CH were used as test materials. Additionally, ultrafine-grained metal were fabricated by Equal-Channel Angular Pressing method using a die having channel, φ=90°, Ψ=15 °, from semi-solid AC4CH alloy. And then, cylindrical housings with 9.4mm in diameter and 0.5mm in thickness were formed from as-cast materials and ECAPed samples by impact backward extrusion at room temperature. As a result, press load required for extrusion was decreased using a material processed with increasing the number of ECAP. Vickers hardness of the products was greater than that of the materials before the forming; in the case of using ECAPed materials, the increment was relatively small. This implies that impact extrusion process is a kind of Severe Plastic Deformation that improves mechanical properties of metal materials. Ultrafine-grained semi-solid AC4CH alloy have more possibility to make the housings with further thin thickness than non-ECAP materials.