Adsorption of sodium salt of poly (acrylic) acid (PAANa) on nano-sized CaCO3 surface was studied by using potentiometric titration. The dispersion of nano-sized CaCO3 in water was also studied, and the viscosity of nano-sized CaCO3 slurry was measured. The results indicate that the adsorption of PAANa on nano-sized CaCO3 is significantly affected by the pH of the slurry, and the saturation adsorption amount decreases as pH value increases. The dispersion of nano-sized CaCO3 in water depends on the amount of added PAANa and also on slurry pH value. In the range of the pH values studied, the stronger the alkalinity of nano-sized CaCO3 slurry, the better is the dispersion of nano-sized CaCO3 modified with PAANa. Moreover, there exists an optimum amount of the added PAANa corresponding to the lowest viscosity of the dispersion. In short, PAANa is an effective dispersant to improve the dispersion of nano-sized CaCO3 particles in water.