The synergistic effects of some ions such as KI ,KCl and KBr on the corrosion inhibition of zinc in 0.4 M HCl by tetrahydrocarbazole derivatives compounds has been investigated using weight loss measurements. Potentiodynamic anodic polarization technique was used to measure the pitting corrosion potential and Quantum Chemical studies (Computational Study) were investigated. Results obtained show that these compounds are good inhibitors and their inhibition efficiencies (IE %) increase with the increase of inhibitor concentration .The presence of I-, Br- and Cl- anions increase the inhibition efficiency of tetrahydrocarbazole compounds due to the synergistic effect .Halide ions are normally strongly adsorbed on the zinc surface and facilitate the adsorption of organic compounds by forming intermediate bridges between the positively charge metal surface and the positive end of the tetrahydrocarbazole molecule .Thus, the inhibitor is not adsorbed directly on the metal itself but rather by columbic attraction to the adsorbed halide ions on the metal surface.It is also found that these compounds inhibit the pitting corrosion of zinc in chloride containing solution by shifting the pitting corrosion potential into more positive(noble )direction. Quantum parameters chemical parameters were constructed in order to characterize the inhibition of these compounds efficiency.