The molecular orientation behavior of mesomorphic isotactic polypropylene films under biaxial extension was investigated at room temperature. The birefringence was measured not only for drawn films but also simultaneously with biaxial stresses. It was found that there exists a yield point at which the neck is initiated on the biaxial deformed films and necking occurred and propagated in only one stretching direction, resulting in mosaic structures consisting of necked blocks. POLYM. ENG. SCI., 51:225-231, 2011. (C) 2010 Society of Plastics Engineers