PP samples containing beta-phase crystals were prepared by doping with a beta-nucleating agent. The beta-PP specimens were tensile stretched at room temperature under various crosshead rates and the yielding behaviour was analyzed on the basis of Eyring＇s theory. After tensile testing, the beta-phase was examined with WAXS and D.S.C. over the necking region and the deformation mechanisms were discussed. The beta-phase was found to be mechanically stable at room temperature up to yielding point. However, once the samples were stretched to necking, the beta-phase began to transform to the alpha-phase; this phase transformation continued over the necking region. The phase transformation implied that local melting and crystallization occurred during cold drawing and gave powerful support to Flory＇s model of the deformation mechanism of crystalline plastics.