A statistical model approach called response surface methodology was used to describe the product and substrate inhibition effect on P-galactosidase enzyme during lactose hydrolysis. The effect of independent variables, namely the initial concentrations of lactose (73 - 146 mM), galactose (44 - 122 mM) and glucose (83 - 167 mM) on the reaction rate of P-galactosidase was evaluated. The enzymatic reaction rate was influenced by both combined and individual effects of all the substrate and products. Although, glucose acted as an activator at low lactose and low galactose concentrations, glucose caused the inhibition of beta-galactosidase at higher concentrations of lactose and galactose. The effect of galactose concentration on beta-galactosidase enzyme was in the direction of inhibition. At low lactose concentrations and high glucose concentrations, galactose concentration became more effective on the reaction rate.