Iron sulfide is present in the internal pipeline surface or suspended in liquid phase especially in highly sour environments. The presence of iron sulfide may significantly affect the corrosion inhibition efficiency. The objective of this investigation is to study the effect of iron sulfide on the efficiency of corrosion inhibitor in highly sour environment. Experiments were performed in a series of autoclaves with a total pressure of 130psi (78psi CO2 42psi H2S and 10 psi water vapor). Iron sulfide effects were studied separately in two different cases: directly deposited on the steel surface or suspended in the test solution. In the experiments weight loss and Electrochemical Impedance Spectroscopy (EIS) were applied to study the corrosion inhibition efficiency. Corrosion product films/scales formed on the surface of the steel were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX).