The inhibition efficiency (IE) of Amino Trimethylene phosphonic acid (ATMP) in controlling corrosion of carbon steel immersed in ground water in the absence and presence of Zn2+ has been evaluated by weight loss method. It is observed that the synergistic formulation consisting of 250 ppm ATMP and 10 ppm of Zn2+ has 98% IE. Polarization study reveals that ATMP-Zn2+ system functions as a cathodic inhibitor system. AC impedance study reveals that a protective film is formed on the metal surface.