The rubber modified polystyrene resin composition with high gloss and impact resistance according to the present invention is prepared by (i) polymerizing a solution consisting of about 80 to 95% by weight of styrene (A) and about 5 to 20% by weight of polybutadiene rubber (B) with a clay material at a temperature of about 90 to 150.degree. C. in a first reactor so as to disperse at least two different sizes of polybutadiene rubber particles in the matrix of polystyrene, and (ii) inputting the resulting solution of the step (i) to a second reactor being kept at about 130 to 180.degree. C. to terminate the polymerization.