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The shortcomings of an exit stress variation formula which has been well accepted are analyzed; it is found that the exit stress variation formula violates the premise of the law of volume constancy. The shortcomings of an entry stress variation formula are analyzed too, and the basic assumption of the formula is uniform exit velocity. However, for a rigid-plastic material uniform exit velocity implies that the lateral distribution of elongation is uniform, so the exit stress must be uniform and any type of flatness defect is impossible, which is contrary to the practice. In fact, entry and exit velocity variation influence entry and exit stress variation, and entry and exit stress variation influence entry and exit velocity variation too, so a precise explicit stress variation formula cannot be got easily. Considering the relationship between stress variation and velocity variation, an iteration method is presented to calculate entry and exit stress variation of cold rolling strip. To avoid divergent phenomenon of the iteration course, a relaxation factor method is adopted. The calculation results are compared with the entry and exit stress variation formula commonly used by many researchers. The difference is remarkable, while the result calculated agree more well with measured result if the exit elastic recovery zone is considered. Specially, the incoming flatness′ propagate efficiency calculated gives a more realistic result.