Miscrible displacement can be understood as a physical process in a porous media whereby two or more fluids fully dissolve into each other when a fluid mixes and goes into the pore space occupied by other fluids without the exixtence of an interface. A physical model was made in Can Tho University, which included an electrical current system connecting nine groups of four-elctrode probes for measuring the electricity conductivity of a potassium chloride solution flowing through a horizontal sand column placed in a firm frame. The experiments were performed with different volumetric flow rate and three types of sand (fine, medium and coarse). The breakthrough curves were analysed, and then the hydrodynamic dispersion coefficients were calculated. The hydrodynamic dispersion coefficent was one of the hydralic and solute transport parameters used to design a constructed subsurface flow wetland. The research proves that the flows were lanimar, and that mechanical dispersions dominated over molecular diffusions and that the dispersions were large enough to cause combined mixing and flowing processes.