The effect of the horizontal dimensions of inland water bodies (lakes and reservoirs) on the vertical mixing processes is studied. We consider a three-dimensional hydrostatic model and a one-dimensional LAKE model based on the averaging of three-dimensional equations over a horizontal section of the water body. The processes of vertical mixing in both models were simulated with the use of k–e-closure. LAKE model takes into account the seiches through the parameterization of the pressure gradient and horizontal viscosity. The models were verified against the Kato-Phillips experiment data, and a series of numerical experiments was carried out to demonstrate the effect of the horizontal size of the water body on mixed-layer depth. It is shown that the horizontal dimensions of a water body has to be taken into account in the simulation of the vertical temperature distribution in lakes and reservoirs with the size much less than the internal Rossby deformation radius.