Abstract: The isothermal crystallization behavior of cis-1, 4polybutadiene raw rubber was studied at depressed or elevated temperature by means of a linear dilatometer. It was found that the length of some gel-containing samples increases at the beginning of crystallization, and it decreases until the crystal is melted completely. These abnomal phenomena disappear gradually with the process of annealing of the sample. In this paper a model of the gelcontaining sample is proposed which may preserve the internal stress by adding an external force. Owing to the internal stress preserved in the sample, the oriented gel parts form a fibrillar crystal at first stage, which lengthens the sample,then the spherical crystal formed in the sol parts reduces the length of the sample. Such interpretation is further confirmed by DSC experiments.