The effect of electrostatic field on apparent kinetic parameters in a packed-bed immobilized enzyme reactor was investigated using a mathematical model. The relationship between the dimensionless concentration of substrate consumed in the reactor and the logarithm of the unconverted fraction of substrate at the reactor outlet was nonlinear and changed remarkably according to the magnitude of electrostatic potential and the sign of charges that carry the support and substrate. For the opposite sign of these charges, the apparent Michaelis constant was less than its intrinsic value in the region of high substrate concentration, which reflects a promoting effect on the enzyme reaction by the electrostatic attraction.