This invention produces heat resistant expandable plastic particles by the method of emulsion polymerization, containing .alpha.-alkylstyrene 15-55 weight %, another polymerizable monomer 85-45 weight %, and a readily volatile blowing agent 3-15 weight % as chief ingredients, and offers the expandable plastic molded articles with high heat resistance and high dimensional stability produced by prefoaming the expandable polymerizate particles with heating at a pressure of more than 0.1 kg/cm.sup.2 G followed by heat-molding in a metallic mold.