General polymeric microspheres are not satisfactorily thermostable. This article reports on an unprecedented type of poly(N-phenylmaleimide-co-alpha-methyl styrene) [denoted as poly(N-PMI-co-AMS)] microspheres showing remarkable thermal stability. The microspheres were prepared by free-radical precipitation polymerization in a solvent mixture consisting of methyl ethyl ketone (favorable solvent) and heptane (unfavorable solvent). Microspheres of good morphology and narrow size distribution were obtained in high yield (>85%) under appropriate conditions. Growth of poly(N-PMI-co-AMS) microspheres was characterized by scanning electron microscopy. The microspheres, although without cross-linking, exhibited excellent thermal stability, and their decomposition temperature was up to about 370 degrees C. This feature cannot be achieved in typical polymeric microspheres. Also, notably, this is the first precipitation polymerization of maleimide and AMS and their derivatives for preparing microspheres. The present novel microspheres are expected to find practical applications as novel heat-resistant additives, solid carriers for catalysts, and so on.