The objective of this study is to explore the structure evolution of isotactic polypropylene (iPP) during creep under sub-yield stress by in-situ synchrotron small angle X-ray scattering (SAXS). Generally, creep curve of iPP at relative high force can be split into four regions. In region I, strain grows nonlinearly to a value smaller than 15 % at sufficiently small times. In this region, only the long period along drawing direction (La) increases with the increase of time, whereas the long period perpendicular to drawing direction (Lp) keeps constantly. The increase of La is caused by elastic deformation of the non-crystalline phase in the polar region of iPP spherulites. In region II, strain linearly increases with time, and both La and Lp increase as time increases. Two possible reasons are proposed for the increase of long period. One of them is annealing induced crystalline perfection by melting-recrystallization and the other one is strain-induced increase in free volume. In region III, strain grows dramatically with time, and the yielding of iPP occurs in this region. In this region, the peak intensity positions of the lamellae reflection in 2D-SAXS patterns change from a circle or light ellipse to a distinct ellipse. This significant transition indicates that the lamellae is tilted and rotated during the yielding process. In the last region, strain grows linearly with time again. The lamellae reflection grows from distinct ellipse into two spots, which means that lamellae aligned mainly along the drawing direction after yielding. What’s more, a streak perpendicular to the drawing direction shows up in this region. After evaluating the intensity of the streak, it is proposed that the streak is a signal of shish, instead of cavity. Employing the Ruland’s streak method, it is calculated that the length of shish is ca. 10 nm and grows to ca. 15 nm with the increase of time.