As　an　important　branch　of　quantum　cryptography,　quantum　private　comparison　(QPC)　has　recently　received　a　lot　of　attention.　In　this　paper　we　study　the　security　of　previous　QPC　protocols　with　a　semi-honest　third　party　(TP)　from　the　viewpoint　of　secure　multi-party　computation　and　show　that　the　assumption　of　a　semi-honest　TP　is　unreasonable.　Without　the　unreasonable　assumption　of　a　semi-honest　TP,　one　can　easily　find　that　the　QPC　protocol　(Tseng　et　al.　in　Quantum　Inf　Process,　2011,　doi:10.1007/s11128-011-0251-0)　has　an　obvious　security　flaw.　Some　suggestions　about　the　design　of　QPC　protocols　are　also　given.