Tap kinds of styrene microemulsion systems were polymerized to prepare polystyrene microspheres (micro-PS), one emulsified by sodium dodecyl sulfate (SDS) and coemulsified by 1-pentanol and the other emulsified by octylphenoxypoly(ethoxyethanol) (OP-10). Micro-PS possessed an essentially atactic configuration hom H-1-NMR measurement. Comparison of LR spectra showed some conformational differences between micro-PS and ordinary atactic PS (o-PS). Each polystyrene microsphere contains only about three macromolecular chains iso-called pauci-chain microspheres!? which adopt a special conformation : individual chains are highly compact, while the whole structure of the microsphere is relatively loose in comparison with a random-coil chain in a multichain system. There were apparent differences in thermal behaviors between micro-PS and o-PS during the DSC scan. Micro-PS displayed two exotherms near 107 and 157 degrees C during the first scan. In the subsequent scan, both exotherms disappeared and only one glass transition similar to o-PS was shown. The first exotherm might be due to the sintering of polystyrene microspheres with small particle sizes. From wide-angle X-ray diffraction (WAXD) and polarizing optical microscopy (POM) studies, the second exotherm could be ascribed to the relatively compact conformation of polystyrene chains and the consequently formed slightly ordered regions inside micro-PS during the heating process. The special conformation was the driving force leading to its novel thermal behaviors. This is perhaps the first report that polystyrene from direct free radical polymerization has the ability to form some kind of ordered structure.