PURPOSE: Expandable resin particles having excellent heat resistance, comprising α- alkylstyrene, a polymerizable monomer and an easily volatiel blowing agent.

CONSTITUTION: 15W55wt% α-alkylstyrene, where alkyl is methyl, ethyl, n-propyl or isopopyl and 85W45wt% polymerizable monomer (e.g. styrene) are emulsionpolymerized in an aqueous medium in the presence of a radical polymerization catalyst, a molecular weitht modifier and an emulsifier, and then salted out to be solidified and dried. The resulting powdery polymer is fed to an extruder to be pelletized. Subsequently, the pellet, an emulsifier (e.g. PVA) and water are charged into an auto-clave and heated. To the heated mixture, 3W15wt% easily volatile bolowing agent (e.g. n-butane) is added to have the polymer impregnated with it. Thus, expandable synthetic resin particles having a specific viscosity (Ostwald viscometer, a polymer concentration of 1% in toluene, 30±0.1°C) of 0.6W1.7 are obtained.