When TiO2 particle doped PVDF sheets are irradiated with UV-light, the sheets become cloudy, because local photocatalytic decomposition of PVDF leads to an increase in the surface roughness on a sub-micrometer scale. This is called "chalking". Chalking results in decreased photocatalytic activity of the sheet. In this study, we investigated the chalking process of TiO2 doped PVDF sheets using SEM/EDX and AFM. In the first stage of chalking, minute cavities appear everywhere in the PVDF sheet surface. With UV-light irradiation, it is found that the density of minute cavities increases until they are connected laterally and then grow into minute cracks. When the decomposition of the PVDF progresses, TiO2 particle aggregates, which show cracks, are exposed on the surface. These results suggest that tensile stress builds up because of the PVDF decomposition.