Polymer processing generally plays a crucial role in determining the development of microstructure in the fabricated product. In this paper, isotactic polypropylene (iPP) containing 0.1 wt% beta-nucleating agent was extruded via a slit die and immediately melt-stretched upon various stretching rates (SR) at the die exit. The microstructure of beta-nucleated iPP samples was investigated using two-dimensional wide-angle X-ray diffraction and small-angle X-ray scatterings (2D-WAXD/SAXS) measurements. It is observed that, no matter what SR is, lamellar branching of beta-form crystal is formed in melt-stretched samples, which is scarcely reported in open literatures. A method is proposed to calculate the crystallinity of daughter lamellae in beta-crystal (X beta-D). It is found that X beta-D decreases with increasing SR, indicating that lamellar branching of beta-form crystal is restrained by higher SR. Such case should be attributed to the denser-oriented structures (e. g., shish) induced by higher SR, leaving a confined space between adjacent-oriented structures for the growth of daughter lamellae.