In present work, five 1,3,4-thiadiazole compounds were synthesized. The prepared compounds were identified by CHNS analysis, FT-IR and 1 H NMR spectroscopy. The corrosion rates in the presence of thiadiazole as a steel corrosion inhibitor in the cooling water system which taken from South Fertilizer Company, Basra, Iraq were measured by the weight loss method and potentiodynamic polarization measurements. The weight loss method was studied in different times (1-5 h) and in 303 to 333 K temperature range. Results obtained revealed that thiadiazole compounds performed as a corrosion inhibitor for mild steel in this medium and its efficiency attains to 86.55 % at 5 × 10 -3 M at 303 K and by potentiodynamic polarization measurements its efficiency attains to 85.35 % in the same conditions. The apparent activation energies, enthalpies and entropies of the dissolution process and the free energies were determined and discussed.