The efficacy of an acid extract of leaves of Sida rhombifolia.L as corrosion inhibitor for mild steel in 1M phosphoric acid medium, was carried out using weight loss measurements, polarization and electrochemical impedance spectral studies. Experiments are performed by varying immersion period, concentration of the inhibitor and temperature. The results showed variation in inhibition performance of the inhibitor with varying concentration, immersion time and temperature. The maximum efficiency was found to be 97.48% at 1 %( w/v) concentration of the inhibitor for the immersion period of 24 hours. Langmuir and Temkin were tested to describe the adsorption behavior of inhibitor on the mild steel surface. Potentiodynamic polarization study clearly revealed that inhibitor acts as a mixed type of inhibitor. The results of the electrochemical impedance study showed a decrease in double layer capacitance as the adsorption of inhibitor caused structural change at electrode-solution interface.