Structure changes of syndiotactic polypropylene (sPP) under uniaxial stretching are studied with the combination of micro-tensile tester and in situ wide angle X-ray diffraction (WAXD) measurement. Lamellae stacked "vertically" and "parallel" to the stretching direction (defined as "V" and "P" part) are separated on the basis of two-dimensional WAXD patterns. For all samples with different lamellar thickness, two critical points named as b (1) and b (2) were found in the stress-strain curves, while b (1) and b (2) points are the onsets of the rotation for the lamellae of "V" part and "P" part, respectively. The corresponding true stress and true strain for b (2) point are bigger than that of b (1) , demonstrating that for samples with initial isotropic lamellar orientation, inhomogeneous deformation of crystalline skeleton induced by uniaxial stretching is universal. And after b (1) point, "stress-induced melting" always occurs simultaneously with lamellar slips. Furthermore, the relationship between lamellar thickness and the true stress for b (1) and b (2) point was also studied, illustrating a linear correlation between ln sigma and 1/l (sigma is the corresponding true stress, l is the lamellar thickness), consistent with Young's model. However, the critical true strains for these two points did not change with the varying thickness.