A cylindrical lithium secondary battery includes a wound electrode assembly having a first electrode plate, a second electrode plate, and a separator interposed between the first electrode plate and the second electrode plate. A core member is substantially in a shape of a pipe and has a cut formed along a longitudinal direction thereof, the cut forming two edges. The core member is inserted into a central space of the electrode assembly and has a predetermined deformation direction when subjected to external pressure. The battery also includes a case with a space for accommodating the electrode assembly and an open upper portion, and a cap assembly fixed to the upper portion of the case to hermetically seal the upper portion of the case.