Enzymes are the catalysts of biological schemes and are extremely effective and exact as catalysts. Enzyme substrates are manipulated by protein molecules which are called enzymes. Enzyme kinetics is primarily concerned with the estimation and mathematical recount of this answer rate and its affiliated constants. Single-substrate reactions are Michaelis–Menten kinetics, direct use of the Michaelis–Menten formula for time course kinetic investigation, linear plots of the Michaelis–Menten equation, Practical implication of kinetic constants and Michaelis–Menten kinetics with intermediate. Multi-substrate reactions are Ternary-complex mechanisms and Ping–pong means. Hypertension is due to the activity of the Angiotensin-Converting Enzyme that catalyzes the conversion of the inactive Angiotensin I to the active Angiotensin II. Angiotensin II functions as a potent vasoconstrictor causing hypertension. Hence, ACE inhibitors are considered to be the first choice treatment of hypertension. The ACE inhibitor activity of Psidiumguajava Linn.leaf extract in human was studied by its in vitro inhibitory effect on human serum-ACE using the substrate FA-PGG (N-(3-(2-Furyl) acryloyl)-L-phenylalanyl-glycyl-glycine). The enzyme kinetics of ACE inhibition was investigated by Michaelis-Menten kinetics and Lineweaver-Burk graph and Vmax and Km values were calculated.