Based on a Hartree-Slater self-consistent theory, we calculate two-photon ionization cross-sections of Cs and Xe as illustrative examples. Our theoretical method can easily be applied to calculate other types of two-photon transitions and can also be extended to calculate higherorder multiphoton ionization processes. Comparing our theoretical results with available experimental data and other theoretical results, we can discuss the validity of our theoretical method. For two-photon ionization processes of alkali atoms (e.g. Cs), our results are in good agreement with recent accurate experimental measurements.