From thermodynamic relations, the initial values of the quark-gloun plasma from relativistic nucleus-nucleus collisions are obtained. From these initial values we have studied the dilepton production on the basis of the (3+1) dimensional relativistic hydrodynamic model established by the present authors, and found that with increasing incident energy a characteristic plateau indicating the formation of quark-gluon plasma appears in the total yield, which may be tested in future experiments in CERN and Brookhaven.