Uncertainty theory provides a new tool to deal with the maximum flow problem of uncertain network with the uncertain loss.The maximum flow problem is one of the classic combinatorial optimization problems with many applications.The problem is to find a flow of maximum value on a network from a source to a sink.With help from the operational law of uncertainty theory, this paper gives the uncertainty distribution of the lost flow.The purpose of this paper is to study the maximum flow problem of uncertain network with the uncertain loss by the uncertainty theory.