Abstract: An impact-resistant styrenic polymer resin composition comprising 70-97 wt. % of a styrenic polymer resin and 3-30 wt. % of a toughening agent of a fine particle form having an average particle diameter of 0.3-0.7 .mu.m and dispersed in the styrenic polymer resin. The toughening agent comprises (I) 10-98 wt. % of a styrene/butadiene di-block copolymer having (i) a weight average molecular weight (MW) of 30,000-1,000,000, (ii) a MW distribution of 1.00-1.20 as defined by the weight average MW/number average MW ratio, (iii) a total bound styrene content of 30-70 wt. %, and (iv) a block styrene content of at least 60% based on the total bound styrene content; and (II) 2-90 wt. % of a styrene/butadiene copolymer rubber, polybutadiene or a mixture thereof, which has (i) a weight average MW of 1,000-1,000,000, (ii) a MW distribution of 1.00-1.60 as defined by the weight average MW/number average MW ratio, (iii) a total bound styrene content of 0-15 wt. %, and (iv) a cis-1,4 bond content of 10-80 wt.