Thermal stability of syndiotactic polypropene (syn-PP.) was investigated by comparison with isotactic polypropene (iso-PP). Heat treatments were carried out in air at 160-220 degrees C for 10-30 min. Under these conditions, molecular weight and molecular weight distribution of syn-PP were almost kept constant, indicating the high stability of syn-PP. Iso-PP was degraded drastically under the same conditions. Thus, it is confirmed that the stereoregularity is one of the dominant factors to determine the stability of PP.