Flatness imperfections are observed during uncoiling of the hot rolled strip after forced cooling of coils to ambient temperature. This problem is significant in thin strip as free defect was considered in the process. A mathematical model for heat transfer and thermal deformation analysis during cooling of hot rolled coils is developed to explain the phenomenon of post-rolling flatness deterioration, and predict the occurrence and magnitude of the defects. The numerical simulation results obtained in this work were experimentally validated. It is shown, at a low coiling temperature allowing the phase transformation to reach completion, that the axial distribution of the temperature mainly causes the variation of the local thermal stress, and will promote the flatness deterioration.