A series of β-nucleating agent modified iPP (β-iPP) films were prepared by melt extrusion-stretching. The effect of stretching rates (SR) on microstructure and tensile properties of β-iPP films was investigated in detail. The results show that there is lamellar branching of α- and β-crystal in all films prepared at different SR, and the lamellar branching of β-crystal is inhibited with increasing SR, however the lamellar branching of α-crystal has no significant change; the chain and lamellar orientation degree of β-iPP films increase with increasing SR, but the relative content of β-crystal and its crystallinity exhibits opposite tendency. In addition, tensile test indicates that, when the SR is 30 cm/min, the tensile strength and toughness of β-iPP films reach the maximum.