  In this paper we present a necessary and sufficient condition of
distinguishability of bipartite quantum states. It is shown that the operators
to reliably distinguish states need only rounds of projective measurements and
classical comunication. We also present a necessary condition of
distinguishability of bipartite quantum states which is simple and general.
With this condition one can get many cases of indistinguishability. The
conclusions may be useful in understanding the essence of nonlocality and
calculating the distillable entanglement and the bound of distillable
entanglement.
