PURPOSE: To obtain the title particles excellent in rigidity, adhesiveness, printability, etc., by graft-polymerizing a polymerizable vinyl monomer with hollow PP particles in the presence of a radical polymerization initiator in an aqueous dispersion.

CONSTITUTION: Hollow PP particles (A) are obtained by polymerizing propylene in the presence of a catalyst comprising an α-olefin polymerization solid catalyst component (i) containing Ti and Cl, having a form of a nearly spherical particle of an average particle diameter of 15W80μm, being hollow (the diameter of this hollow pore is at least one tenth of the outside diameter of this hollow pore) and having Ti as an effective component, an organoaluminum compound (ii) and, optionally, an electron-donating compound (iii). 100pts.wt. component A dispersed in 100W2,000pts.wt. aqueous medium and 0.1W200pts.wt. polymerizable vinyl monomer (B) are graft-polymerized for 2W8hr at 50W140°C and a pressure of normal pressure to 15kg/cm2 in the presence of 0.005W5pts.wt. radical polymerization initiator (C) which is an organic peroxide.