PURPOSE: A hydraulic fluid composition that is prepared by adding a phosphoric acid aliphatic alcohol ester and an alkaline earth metal sulfonate to the lubricant base oil at a specific ratio, thus showing high lubricity, high rubber deterioration resistance, low fluidizing point and reduced change with time.

CONSTITUTION: The objective composition comprises 100pts.wt. of the lubricant base oil, (A) 0.1W5pts.wt. of an aliphatic alcohol ester of phosphate or phosphite salt, (B) 0.1W5pts.wt. of an alkaline earth metal sulfonate, (C) 0.2W10pts.wt. of polyalkyl methacrylate, (D) 0.001W1pt.wt. of ethylene-vinyl acetate copolymer, and (E) 0.01W1pts.wt. of paraffin wax.

USE: Oil for hydraulically operating machines such as hydraulic shovels, hydraulic press or injection molding machine, hydraulic oil for air crafts, oil for automatic gear changer for vehicles, brake fluid, shock absorber oil, power-steering oil.