Abstract: Electromagnetic field formulas for horizontal electric dipole and verticale magnetic dipole on layered earth are quoted.Nuclear functions in the formulas are transformed,and wave modes as well as the relations to the fied zone are analyzed.The penetration ability of TE mode,TM mode and TEM mode is indicated.According to the wave mode and feature of its effect upon strata,the electromagnetic field components are selected.Therefore,prospecting result for electromagnetic frequency sounding will be improved.