A co-modified organopolysiloxane having a viscosity at 25° C. of not more than 1,500 mPa·s, a group that has a siloxane dendron structure, a hydrophilic group, and expressed by the following general formula (1): R1aR2bL1cQdSiO(4-a-b-c-d)/2 (1). The invention also relates to a powder treatment agent, a powder in oil dispersion, an external use preparation (particularly a cosmetic composition) composition comprising the co-modified organopolysiloxane. In said general formula (1), R1 is a monovalent hydrocarbon group or a hydrogen atom； R2 is a monovalent hydrocarbon group having from 6 to 30 carbons； L1 is a silylalkyl group having a siloxane dendron structure； and Q is a hydrophilic segment.
