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In this study, the crystallization behavior of oriented isotactic polypropylene during solid-state stretching was investigated by in-situ wide-angle X-ray scattering. Results showed that a new arc appeared on the (040) reflection with the previous arcs remained when the stretching direction was inclined to the initial orientation direction, as a result of stretching induced crystallization. The crystallization process occurred later than the yielding but earlier than the strain hardening. The crystallinity of the newly formed crystal showed a linear dependence on the Hencky strain, and the slope of the plot was more sensitive to the stretching angle rather than the stretching temperature. Lastly, the stress relaxation test showed that the crystallinity was linear correlated to the amorphous phase orientation. The Pearson correlation coefficient was as high as −0.99, suggesting that the crystallization process is originated from the orientation of the chain network during stretching.