A new method for determining the energy gap of a superconductor using the maximum in the differential conductance curve of electron tunneling spectrum is given in this paper. The Vmax/kT versus △/kT curve was caculated from tunneling theory, Vmax, the voltage of the conductance maximum, can be measured from electron tunneling spectrum.△/kT can be found from this curve, then the energy gap △ can be caeulated. This method is simple, fast and accurate. The accuracy almost approaches that of the curve fitting method.