The inhibition impacts of 1,10-Phenanthroline-5,6-diamine (PTDA) on mild steel in 1 M HCl solution were investigated through weight loss method. The inhibition efficiencies of PTDA increase with increase in PTDA concentration at the temperature 303. Weight loss method indicate that PTDA is an excellent inhibitor the inhibition efficiency of 81.5% at the maximum PTDA concentration of 0.5 g/L at the temperature 303K.