The yield stress of Na-montmorillonite suspension in an electrostatically dispersed state was studied to confirm the validity of previously proposed equation for the dependency of the yield stress on the volume fraction of clay and ionic strength. In the derivation of the equation, it is assumed that (1) clay layers array in parallel, and (2) the yield stress is equivalent to the strength of electrostatic repulsion when it reaches the yield stress. Measurements of the yield stress were performed with the vane method, varying the volume fraction of clay, phi, under controlled ionic strength for the region of 0.008<phi<0.023 and 10(-2)<NaCl<10(-5) M. Experimental data qualitatively agreed with theoretical predictions; however, quantitative differences were found to range from 0.1 to 1000 times. This discrepancy can be eliminated if we introduce a correction factor. Reasonable agreement was obtained when the factor was proportional to n(0)(-0.256).