In the present paper an approach for reduction of measurement uncertainty of the dielectric constant of a rectangular slab in a rectangular waveguide is discussed. The experimental model under consideration consists of two rectangular full height and full width slabs located in an otherwise empty section of a rectangular waveguide. The dielectric constant of one of these slabs is to be measured, while the dielectric constant of the other slab is known (measured with high accuracy in advance). The slab with known constitutive properties is introduced for the purpose of altering the shape of the curve representing the relationship between the absolute value of the reflection coeffcient and the dielectric constant to be measured, since curves having larger steepness in the neighborhood of the actual value of the dielectric constant result in smaller values of uncertainty. Although, it is possible to change the shape of the curve by varying sample parameters, the steepness obtained this way is not always suffcient. Moreover, it not always possible to alter the dimensions of the sample under test or change frequency. The results of the present study show that this issue can be overcome by extending the experimental model, i.e., by introducing an auxiliary dielectric slab with known constitutive properties. Additionally, it is shown that under certain conditions it is always possible to increase the steepness of the curve in the range of values in which the value of the dielectric constant is expected to fall, by varying the thickness of the auxiliary slab and distance between the slabs. The effciency of the proposed approach is confirmed by results of numerical modeling.