Summary: We examine the validity of the application of the factorization approximation to derive the master equation for a microscopic system coupled to a reservoir. We developed a formal perturbation expansion for the time evolution of the system reduced density matrix. We employed a diagrammatic schemes to produce each term of the perturbation series. The diagrams in the time domain provide a distinct criteria to distinguish the diagrams which survive the factorization approximation. The Feynmann-like diagrams in the energy domain, originated from the resolvent method, are used for execution of diagram summations to estimate their overall contributions. We demonstrated that for a two level atomic system, interacting with a thermal reservoir, the summation over the diagrams which survived the factorization approximation, yields the proper time evolution of the system, in agreement with the solution of the master equation. The summation of the diagrams which are excluded by applying the factorization approximation are characterized by a dimensionless parameter: \(\Gamma/\omega_0\), where \(\omega_0\) is the frequency of the transition line, and \(\Gamma\) is the line width. The factorization approximation is thus rigorously justified when this expansion parameter is very small.