This article reports on a novel type of polymeric microsphere consisting of poly(N-(1-phenylethyl)maleimide-co-styrene) (poly(N-PEMI-co-St)) and showing remarkable heat resistance. Such microspheres were prepared by dispersion polymerization with 2,2′-azobisisobutyronitrile as an initiator and poly(vinyl pyrrolidone) as a dispersant. The growth of the microspheres was observed with SEM. The effects of some crucial factors on the formation of the miscrospheres were investigated, including molecular weight and mass fraction of the dispersant, monomer concentration, and the initiator mass fraction. Microspheres with good morphology and narrow dispersion were prepared under the optimized conditions. TGA demonstrated the remarkably high thermal decomposition temperature (approx. 450 °C) of the poly(N-PEMI-co-St) microspheres.