Abstract: The effect of sodium humate and its sulfonated product on the electrokinetic property of colloidal particles of bentonite has been studied. The mobilities of the particles were measured with a laser-zeta meter and their values were converted into zeta potentials ξ by using Smoluchowski equation. During the process of mud-making, the mobilities of the particles or |ξ|potential increases with the addition of Na₂CO₃ and the increase in pH as a result of the improvement in the dispersion of the particles and the increase of negative charges on the surface of the particles. In the presence of neutral electrolytes, Nad and CaCl₂,the relation between ξ and the electrolyte concentration can be expressed as follows, ζ=A+BlogNaCl, ξ=A+B(logCaCl₂²; where A=-37.8±0.5 and -9.1±0.5, B=2.0±0.2 and -1.5±0.2, for NaCl and CaCl₂ respectively. With the addition of sodium humate and sulfonated sodiun humate, the |ξ| of the system increses quite significantly and the properties of the processed drilling-mud; such as apparent viscosity, plastic viscosity, yield value, shear stress and filtration loss, are improved.