The two-photon ionization spectrum of Cs atom in a strong electric field wasstudied experimentally.It was found that the core effect of Cs atom due to the higherquantum defects can affect the ionization spectrum well above the field ionization threshold,but has negligible influence on the strong field resonances near the zero field ionizationthreshold.The dependance of the strong field resonance on the polarization of the excitationlaser beam seems not fully consistent with predictions and a further study is needed.