Abstract: The rubber-modified styrene-based resin composition, which is a composite of a styrene-based resin as a continuum of the matrix phase and a diene-based rubbery polymer as a dispersed particulate phase in the matrix, is characterized by the parameters including an average particle diameter of the particles of the rubbery polymer in the range from 0.08 to 1.00 .mu.m, peripheral parameter thereof in the range from 0.1 to 2.5 (.mu.m).sup.-1. (% by weight).sup.-1 and relaxation time T.sub.2 thereof in the range from 300 to 2000 .mu. seconds as the determinant factors of the moldability of the resin composition as well as the impact strength and surface gloss of the shaped articles prepared from the composition.