A surface treatment agent or a powder treatment agent comprising a co-modified organopolysiloxane is disclosed. The co-modified organopolysiloxane is represented by the general formula: R1aR2bQcSiO(4-a-b-c)/2. The co-modified organopolysiloxane has a number average molecular weight of less than 4500 and includes a long-chain alkyl group and a polyoxyalkylene group in the molecule.