Mechanical properties and lithiation-induced stress are crucial to the performance and durability of lithium-ion batteries. Here, we report the evolution of elastic modulus and stress in a silicon/polyvinylidene fluoride (PVDF) composite electrode coated on a copper foil, along with a model for analyzing the large change in the radius of curvature of the composite electrode/copper foil cantilever. The radius of curvature of the cantilever is captured by a video camera during lithiation/delithiation. The elastic modulus of the composite electrode decreases from about 0.64 GPa to 0.18 GPa during lithiation. It decreases further to about 0.10 GPa after delithiation, which is caused by the fracture of the electrode. The magnitude of the compressive stress increases lineally during lithiation and decreases suddenly to reach a steady state value during delithiation.