  The paper [Phys. Lett. A 373 (2009) 3610] by D.-C. Li analyzes the
transformation between two-qubit mixed states by local operations and classical
communication. We show that the proof of the main theorem, Theorem 2.6 in
[Phys. Lett. A 373 (2009) 3610] is not complete. Therefore the generalization
of Nielsen's theorem to mixed states still remains an open problem.
