Lamellar stack in semicrystalline polymers composed of alternatively arranged lamellar crystal and amorphous layers is one of the typical hard-soft laminated nano-composites. Considering the Poisson contraction effects during uniaxial tensile deformation along the normal direction of layers, microbuckling instability of lamellar stacks is first proposed as a new deformation mechanism to trigger the nonlinear mechanical behaviors in semicrystalline polymers. Based on the non-equilibrium process of crystallization and experimental observations, a three-phase structure with lamellar stack (crystal and interlamellar amorphous) and the amorphous matrix is proposed as a deformation unit in semicrystalline polymers. Based on the three-phase model and the proposition of buckling with shear mode, we deduce the theoretical critical strain for the sinusoidal microbuckling through linear stability analysis method. Taking hard-elastic isotactic polypropylene as an example, the theoretically calculated critical strain is in a good agreement with the experimental critical strain at temperatures below a relaxation temperature T-alpha. These results suggest that elastic microbuckling is indeed a possible mechanism to trigger the nonlinear instability, which is different from current plastic deformation models with crystal destruction around yield in semicrystalline polymers.