The paper focuses on the effects of polymorphism on photodegradation of isotactic polypropylene. The starting polymer was modified by a specific α-nucleating agent, 1,3;2,4-bis(3,4-dimethylbenzylidene)sorbitol, by a specific β-nucleating agent, N,N′-dicyclohexylnaphthalene-2,6-dicarboxamide, or their combination. Samples prepared by compression moulding were then exposed to UV-irradiation in the interval from 0 to 240 h. The differences in morphology were reflected in different photooxidative behaviour. Infrared spectroscopy showed that neat polypropylene was the most sensitive to photooxidation and the sample modified solely by the β-nucleating agent was the least sensitive. The remaining two samples exhibited an intermediate sensitivity. Differential scanning calorimetry revealed that the UV-exposure led to gradual changes in crystallization mechanism specifically asserting in individual materials. This behaviour was ascribed to homogeneous nucleation of partly degraded macromolecules. Possible changes of the nucleating agent itself during UV-exposure were also discussed.