The inhibition of mild steel corrosion by 4‐amino‐5‐phenyl‐4H‐1, 2, 4‐trizole‐3‐thiol (APTT) in 2.5 M H2SO4 solution and the effect of hydrodynamic condition on inhibition process were studied. The hydrodynamic condition experiments are simulated by rotating cylinder electrode (RCE). Change of open circuit potential (OCP) with immersion time and potentiodynamic polarization were used to study the effect of hydrodynamic conditions on the inhibition process. Results obtained from changes of open circuit potential (OCP) with immersion time, and potentiodynamic polarization are in good agreement and indicated that the inhibition process was flow velocity dependence.