We report in situ tensile strength measurement of fully lithiated Si (Li-Si alloy) nanowires inside a transmission electron microscope. A specially designed dual probe with an atomic force microscopy cantilever and a scanning tunneling microscopy electrode was used to conduct lithiation of Si nanowires and then perform in situ tension of the lithiated nanowires. The axial tensile strength decreased from the initial value of 3.6 GPa for the pristine unlithiated Si nanowires to 0.72 GPa for the lithiated Li-Si alloy. We observed large fracture strain ranging from 8% to 16% for Li-Si alloy, 70% of which remained permanent after fracture. This indicates a certain degree of tensile plasticity in the lithiated silicon before fracture, important for constitutive modeling of the lithium-ion battery cyclability. We also compare the ab initio computed ideal strengths with our measured strengths and attribute the differences to the morphology and flaws in the lithiated nanowires.