The investigation about determining the efficiency of inhibition of johar flower ethanol extract in the corrosion reaction of A36 steel (ASTM)/SS400 (JIS) in 3% NaCl solution has been done. This research aims to determine the immersion time and concentration of johar flower ethanol extract in inhibiting the rate of corrosion on steel plates with the highest value of inhibition efficiency. Making the inhibitor is done by extracting the johar flowers using the maceration method. The extract was mixed into the 3% NaCl corroding media with variations in concentrations of 0, 1000; 2000; 3000 and 4000 ppm, then immersed steel plate samples with a size of 50 × 20 × 3 mm for the variation of immersion time 5; 10; 15 and 20 days. Corrosion testing is carried out using the weight loss method. The test results showed that secondary metabolite compounds in johar flower ethanol extract could function as a corrosion inhibitor by inhibiting the rate of corrosion on the steel plate. The inhibition efficiency for johar flower ethanol extract was found in the concentration of 3000 ppm with immersion time for 15 days, amounting to 33.49%.