PURPOSE: To obtain titled polymer beads of high uniformity free from flocculation, by carrying out a radical polymerization following impregnation in an aqueous medium at a specific temperature, of polyolefin beads with a styrene monomer.

CONSTITUTION: (A) 100pts.wt. of polyolefin beads (e.g., of low-density polyethylene, ethylene-vinyl acetate copolymer) is dispersed in an aqueous medium, being impregnated with (B) 5W200pts.wt. of a styrene monomer at such a temperature that, when the beads (A) are immersed in a large excess of the monomer (B), the amount of the monomer (B) impregnated in the beads (A) may be 50W200wt% based on the beads (A) after a 20min immersion, followed by adding 0.01W5wt% based on the component (B), of a radical initiator (e.g. benzoyl peroxide) to carry out a polymerization, thus obtaining the objective modified polyolefin beads, which is impregnated with a foaming agent (e.g. dichlorodifluoromethane) to obtain the corresponding expandable beads.