A surface-treated powder powder which is surface-treated with an acrylic/silicone copolymer having at least one hydrolyzable silyl group in the molecule, and a cosmetic material containing this powder.The surface-treated powder of this invention preferably contains 0.1-30 wt parts of the acrylic/silicone copolymer relative to 100 wt parts of the powder prior to surface treatment. The acrylic/silicone copolymer preferably comprises a trunk polymer comprising an acrylic polymer, and a branched polymer comprising an organic polysiloxane.The surface activity of the surface-treated powder of this invention is very well blocked, and it has excellent water resistance and sebum resistance. In addition, the cosmetic material of this invention containing this surface-treated powder has good cosmetic lasting properties, and is highly stable over time.