The inhibition efficiency of three substituted phenyltetrazole compounds, namely 5-(4-chlorophenyl)-1H-tetrazole (Cl-PT), 5-(4-methoxyphenyl)-1H-tetrazole (MO-PT), and 5-phenyl-1H-tetrazole (PT), for mild steel corrosion in 5.0 M HCl solution was investigated using weight loss measurements and electrochemical study. It is showed that these compounds are good inhibitors for mild steel corrosion in 5.0 M HCl solution which act as cathodic type inhibitors. So, the inhibition efficiency was increased with inhibitor concentration in the order of Cl-PT > MO-PT > PT which depended on their molecular structures. It is found also that these compounds toke its performance at different temperatures range from 298 to 328 K. In addition, the thermodynamic adsorption parameters at different concentrations were investigated and discussed. Finally, it is seen that the inhibition efficiency of the studied compounds increased with immersion time to reach a maximum at 12 h.