Synchronized by being triggered with the same spark gap, two independently tunable flashlamp-pumped dye lasers with compound cavity structure were used to obtain saturated doubly resonant (3S-3P-4D) three-photon ionization of sodium. Eate equations of such a system under the condition of saturated transitions were analytically solved. Comparison of the calculated ionization curve with experimental results gave good correspondence. The ionization cross section of sodium 4D state has been measured from the cuarve at laser wavelength 689.0 nm σ_(4D)=(589.0 nm)=(1.2士0.4)×10—18 cm2. It is in good agreement with, the theoretical calculation, which is also given in this article.