A novel silicone surfactant comprises a co-modified organopolysiloxane copolymer having a long chain alkyl group, a polyether group, and a specific average polymerization degree of its siloxane back-bone. The silicone surfactant can be used for an emulsifier (especially for a water-in-oil (W/O) type emulsifier), a surface treatment agent, a powder treatment agent, or a dispersant in a cosmetic composition or a medicament. A W/O emulsion composition comprising the silicone surfactant; a powder composition where the surface of the powder is treated using the silicone surfactant as a powder treatment agent; a powder composition further comprising an oil agent and having a form of powder in oil dispersion; and a preparation for external use, particularly a cosmetic composition or medicament, comprising the same; are also disclosed.