To obtain a rosin derivative capable of exhibiting an excellent plasticization effect, an excellent flame retardation effect and an excellent smoke-reducing effect, and useful as a flame-retardant plasticizer for plastics by condensing a rosin alcohol with a specific diaryl phosphate halide.

This rosin derivative is obtained by condensing (A) a rosin alcohol [for example, (dehydro, dihydro or tetrahydro)abietyl alcohol) with (B) a compound of the formula [X is Cl or Br; R is a 1-4C alkyl; (n) is 0-3] (for example, diphenylphosphate chloride). The condensation reaction is preferably carried out in the presence of a basic substance. The rosin derivative is, for example, used in an amount of about 10-200 pts.wt. per 100 pts.wt. of a plastic.