The inhibition efficiency (IE) of carboxymethyl cellulose (CMC)-Zn2+ system in controlling corrosion of carbon steel in ground water in the absence and presence of Zn2+has been evaluated by weight loss method. The formulation consisting of 250 ppm CMC and 50 ppm Zn2+ has 98 % IE. A synergistic effect exists between CMC and Zn2+ . Synergism has been confirmed by synergism parameter. AC impedance spectra confirm the formation of protective film on the metal surface. The nature of the protective film has been characterized by scanning electron microscopy (SEM)