PURPOSE: A flame-retardant hydraulic oil composition that contains an organic phosphate ester, polyester and epoxy compound, thus increasing remarkably flame reatardancy, having high viscosity index, preventing the deterioration, and being usable for long periods of time.

CONSTITUTION: To (A) an organic phosphate ester of formula I or II (R1, R2, R3 are alkyl; R4 is 4W12C alkyl) are added (B) a polyester from 3W24C aliphatic dibasic acid and 2W10C alkylene glycol or a product obtained by terminating the carboxyl or hydroxyl groups of the above polyester on the chain terminals with 6W20C fatty acid or aliphatic alcohol, of which molecular weight is 500W50000 and the amount is 0.5W10wt%, and (C) an epoxy compound in an amount of 0.5W 10wt%. The organic phosphate ester is preferably of formula I as triarylphosphate and the most preferred compound is tricresyl phosphate.