A theoretical model is presented to describe kinetics of the plasma formation in cesium undergone to resonant laser excitation (D1;2 line). The model is based on a rate equations approach where the following populations are considered: ground state (6s level), laser excited level (6p), a series of high excited levels close to the ionization limit, and the electron density. We show temporal evolution of these populations and provide an explanation of the kinetic governing the ionization path-ways. Moreover, we compare the behavior of the electron density as a function of the laser power with the experimental data by Hunnenkens et al. This comparison for the electron density with irradiation time 40 ns is proved a good agreement with the experimental results.