The concept of transition cross section is re-examined in two-photon process. The rate equations and the relevant transition cross sections are good approximation in the region where the diagonal elements of the density matrix change slowly with time. The rate coefficients are derived explicitly:those describing formally the single photon process will be subject to a significant modification for strong light field compared with that of the weak laser field and those describing formally the multi-photon process can also be constructed from the parameters relevant to the real single photon process. The optimization of power distribution of laser fields changes dramatically from weak to strong total laser intensity available. To describe the interference effect originated either from the multi-mode laser field or from a group of energy levels located closely needs more complicated computation of the transition cross sections than using the Bloch equations directly.