The applicability of botanical abrasives to metallic materials was examined by subjecting various metal pieces to barrel polishing with a walnut shell botanical abrasive or a walnut shell-based alumina-composite botanical abrasive. The alumina-composite abrasive was prepared by mixing walnut shell with alumina powder, aliphatic acid, mineral oil, surfactant, and water. 7 kinds of metals were used: aluminum, copper, brass, aluminum alloy, duralumin, stainless steel, and titanium alloy. The applicability of botanical abrasives to metallic materials was verified, with metals with lower hardness or higher density polished more easily. The alumina-composite botanical abrasive was superior to the walnut shell botanical abrasive in polishing performance. The mechanism of barrel polishing is discussed on the basis of the obtained results.