An oil resistant rubber modified polystyrene composition comprises polybutadiene rubber particles having an average volume particle diameter of 6 to 13 nullm dispersed in polystyrene. The oil resistant rubber modified polystyrene of the present invention has a gel content of 25 to 35% by weight, a degree of swelling of 13 to 22, a residual volatile content of 2000 ppm or less, and a total amount of dimer and trimer of 8000 ppm or less. The oil resistant rubber modified polystyrene composition may be prepared by polymerizing styrene in the presence of polybutadiene rubber, foaming a solution of the rubber modified polystyrene under a reduced pressure of 3 to 40 mm Hg, heating the solution in a tubular heater to a temperature of 190 to 230null C., and then flushing the heated solution into a gas-liquid separator to devolatilize an unreacted monomer and a solvent therefrom.