As molecular Rydberg states are very sensitive to external influences, various effects are expected to appear when an electric field is applied. Lowering of ionization threshold, rotational autoionization [1], stabilization of high n levels in vibrational autoionization processes, molecular Stark structure [2] have been recently reported. We present here the Stark structure observed in photoionization spectra of Na2 molecules in an electric field varying up to 6.5 kV/cm.