The stimulated Raman scattering grow up is determined using distributed light scattering model in the single mode and large mode optical fibers. .In the presented model, the pump and Stokes rate equations with considering the thermal noise powers along the fiber length with loss and without loss is solved numerically. Threshold power is obtained by a differential equation similar to the standard threshold relations first derived by Smith. As well as, the diagrams of pump and Stokes powers along the mentioned fibers length is interpreted