The two-photon resonance ionization experiment for the detection of ultra-low concentration of atomic cesium is described. Complete saturation of photoionization was achieved. The number of atoms detected with each laser pulse was measured to be 5.3?l04. By using a tunable dye laser we have achieved the two-photon resonance ionizations the first photon excited cesium atom from ground state 62S1/2 to the excited state 72P1/2, and the second photon ionized the excited cesium atom.