To provide a lubricant additive composition satisfying required performance: to be accepted with pump performance, to satisfy varnish/sludge control, to be water resistant, to be thermally stable and to be environmentally accepted as a lubricant additive for multi grade hydraulic fluids.

Provided is a lubricant additive composition which comprises at least one kind of thermally stable dithiophosphates prepared by reacting a dithiophosphate with an alkene and at least one kind of poly(alkyl methacrylate)-based viscosity index-improving agents, and presents about 300-about 700 ppm phosphorus content when added to a lubricant composition.