Thermodynamics and kinetics of the inhibitory behaviour of Napoleonae imperialis leaves extract on the corrosion of mild steel in 0.5MHCl solution were examined using gravimetric technique at different concentrations of the extract and temperatures (303, 313 and 323 K). Results show that increase in extract concentration and temperature increased the inhibition efficiency of the extract. However, inhibition efficiency decreased with time of immersion of the mild steel coupons. Activation energy of adsorption values obtained at different extract concentrations were positive at all concentrations but higher than that of the blank. Thermodynamic and kinetic data obtained conform to Langmuir adsorption isotherm. The adsorption mechanism suggests physisorptive adsorption process and aligned with the first order kinetics. The highest extract concentration (2.5g/l) showed maximum inhibition efficiency of 84.15% at 323K. The overall results suggest that the leaves extract of Napoleonae imperialis acts as a good inhibitor for the corrosion of mild steel in 0.5MHCl solutions.