The corrosion inhibition of 3-amino-1,2,3-triazole (ATA) on the mild steel in phosphoric acid solution was studied using the weight-loss method and potentiodynamic polarisation and EIS measurements. The addition of ATA decrease the corrosion rate (CR) and consequently the inhibition efficiency increases with concentration. Examination of polarization curves indicates that ATA acts as a mixed-type inhibitor. ATA retards also the cathodic reaction with changing the mechanism of the hydrogen evolution reaction only at 10-2 mol·L−1. The adsorption of ATA on the steel surface obeys Temkin isotherm adsorption model. The free enthalpy energy has been evaluated.