In this paper coherent excitation of two-photon resonance in three-level systems is discussed. Under the condition of two ultrashort pulses with the same waveform, analytical solution of Blochequations of two-photon resonance, level-population and expression of excitation rate in three-level systems have been derived. Conditions for full inversion of population and maximal excitation rate have been obtained. With average-field approximation, analytical solutions of Bloch equations of two-photon off-resonance have been obtained. The analytical expression of isotope coherent selective excitation coefficient has been deduced* Theoretical calculaiiioiis show that the coherent excitation of two-photon resonance has a very high Isotopic selectivity.