The Szeged index of a connected graph G is defined as(Sz(G)=Σe=uv∈E(G)nu(e|G)nv(e|G)),where E(G)is the edge set of G,and for any e = uv ∈ E(G),nu(e|G)is the number of vertices of G lying closer to vertex u than to v,and nv(e|G)is the number of vertices of G lying closer to vertex v than to u.We characterize the graph with smallest Szeged index among all the unicyclic graphs with given order and diameter.