The effect of Lecaniodiscus cupaniodes extract as a green inhibitor on the degradation behaviour of normalized and annealed mild steels in 0.5 M hydrochloric acid (HCL) was evaluated. Gravimetric and potential measurements were used for the experimental work. The results revealed that Lecaniodiscus cupaniodes performed effectively at inhibiting the rate of corrosion attack on the heat-treated steels in 0.5 M HCl medium with an average inhibition efficiency of 90 percent (%). However, the effectiveness of the extract as an inhibitor was found to be dependent on the exposure period for the annealed samples. Corrosion rates of the two heat-treated steels were significantly affected by the concentration of lecaniodiscus cupaniodes extract. At the end of the 34-days exposure period, only 5 ml concentration of the extract gave the best inhibition efficiency for the annealed and normalized steels - 77.09 % and 82.57 % respectively. The significance of the results was further determined via two-factor Analysis of Variance (ANOVA) test. The statistical analysis confirms the effect of extract concentration and exposure time on corrosion behaviour with 95 percent confidence.