A theoretical equation, from which we can get a suitable ratio of bentonite at bentonite-mixed soil, was derived for desigri of the impermeable condition. Bentonite is a soil with great expansion property and it has the permeability lower than $1 imes10^{-7}cm/sec$ in spite of its maximum expansion state. Accordingly if the void of soil is filled with the liquid of bentonite, water will flow only through the veid of bentonite liquid. And the permeability of bentonite-mixed soil will always satisfy the condition as impermeable zone. However, because it is very difficult to mix uniformly bentonite with soil, it is thought that the actual mixing ratio fur the impermeable zone will be grater than that by theoretical equation. Permeability tests were performed to check the equation and a modified equation was suggested from the experimental results.