Currently, various radiative transfer (RT) schemes use their own number of layers in a plane-parallel atmospheric model to simulate the brightness temperature (TB) without considering the effect. The purpose of this letter is to understand numerically and analytically the TB uncertainty due to the layer number of the plane-parallel atmospheric model in RT schemes. Methodically, a plane-parallel atmospheric model in light rainfall and two RT schemes are used. The simulated TBs show a dependence on the number of layers in a plane-parallel atmosphere, which is due to the optical thickness. This letter suggests that the vertical resolution should be considered as a parameter in the forward model of RT schemes.