The rotator phase transition in n-alkane single crystal was investigated mainly by means of in situ optical microscopy. It was found that ＇wrinkles＇ appeared on heating at a temperature slightly below the transition point, and that the rotator phase grew from the wrinkle. It is proposed that the appearance of the winkle is a precursor of the transition. The nucleus of the rotator phase was considered to be formed in the wrinkle. The nucleation rate of the primary nucleus of the rotator phase was also measured. The primary nucleation rate was found to be proportional to exp(-C/Delta T-2), which means that the primary nucleus is three-dimensional one. It was concluded from the results of the morphological observation and the consideration of the observed C value that the primary nucleus is heterogeneously formed in the precursor winkle and that the rotator phase transition is controlled by the nucleation and growth. On cooling, on the other hand, no precursor was observed prior to the transition, and significant large supercooling was observed. This type of hysteresis is commonly observed in the first-order phase transitions in materials. It was shown that the origin of the hysteresis has close relation to the mechanism of the primary nucleation. A universal model of the origin of the hysteresis was proposed, and the tangible evidence of it was shown in the case of n-alkane.