Nonlocality without entanglement is an interesting field. A manifestation of quantum nonlocality without entanglement is the local indistinguishability of a set of orthogonal product states. In this paper we analyze the character of operators to distinguish a set of full product bases in a multi-partite system, and show that distinguishing perfectly a set of full product bases needs only local projective measurements and classical communication, and these measurements cannot damage each product basis. Employing these conclusions one can discuss local distinguishability of full product bases easily. Finally we discuss the generalization of these results to the locally distinguishability of a set of incomplete product bases. PACS number(s): 89.70.+c, 03.65.ud Typeset using REVT E X * E-mail: pxchen@nudt.edu.cn 1 An important manifestation of quantum nonlocality is entanglement [1]. The entangled states can be used for novel forms of information processing, such as quantum cryptography [2,3], quantum teleportation [4], and fast quantum computation [5]. However, there also exists nonlocality in inentangled states [6,7], even in a state of a particle [3]. This is known as nonlocality without entanglement. The protocols of single photon cryptography [3] are examples which uses the nonlocality without entanglement. The nonlocality without entanglement may be an important field just as the entanglement. Closely related to the nonlocality without entanglement is the local distinguishability of a set of inentangled states [6,7].Alice, Bob and Charles et al share a quantum system, in one of a known set of possible orthogonal states. They do not, however, know which state they have. These states are locally distinguishable if there are some sequence of local operations and classical communication (LOCC) by which Alice, Bob and Charles et al can always determine which state they own. There are many interesting works on the local distinguishability of orthogonal states [6][7][8][9][10][11][12][13][14][15]. These works improve our understanding on nonlocality. The discussion on the local distinguishability of orthogonal product states (OPSs) may enlarge our acknowledge of nonlocality without entanglement. Bennett et al first [6] showed that there are 9 OPSs in a 3 ⊗ 3 system which are indistinguishable by LOCC. Walgate et al [7] provide a more simple proof of indistinguishability of the Bennett's 9 OPSs. However few papers discussed the local distinguishability of more general OPSs in a multi-partite system. This paper will focus on the local distinguishability of a set of complete OPSs {|Ψ k } in a multi-partite system. We will show that a set of full OPSs are LOCC perfectly distinguishable if and only if these OPSs are distinguishable by projective measurements and classical communication, and these measurements cannot damage each state |Ψ k . Using this result we can prove easily that the Bennett's 9 OPSs [6] are indistinguishable by LOCC, and can provide many new sets of locally indistinguishable OPSs in multi-p...