Microfiltration with in-line rapid pre-coagulation for the production of drinking water was examined. The in-line rapid pre-coagulation was conducted by using an instantaneous flash mixer. The instantaneous flash mixer injected coagulant into the raw water flow using pressurized supplementary water, with a flow rate ratio to that of raw water set at 0.1. The progress of coagulation was dominated by the charge neutralization with in-line rapid coagulation, because there was no change in the particle size between the raw and coagulated waters, although better coagulation efficiency was observed with coagulant. The flux decline of the microfiltration membrane was investigated with in-line rapid pre-coagulation using different coagulant concentrations. The flux decline of the microfiltration membrane was minimized with a 1.1 mg/L coagulant dosage, where the efficiency of coagulation was maximized and the streaming potential of pre-coagulated water neared zero. These phenomena could be interpreted using the dimensionless distance (K (a) over bar) of particle pairs. which are obtained from electrophoresis parameters that describe the electrostatic repulsion relative to the Van der Waals energy between particle pairs in the pre-coagulated water. As the streaming potential of pre-coagulated water was close to zero, the distance (K (a) over bar) of the particle pairs became too short, allowing easy aggregation due to the minimization of the electrophoretic repulsion between the particle pairs. Consequently, the streaming potential of pre-coagulated water could possibly be used to indicate the optimum coagulant dosage to reduce the flux decline of the membrane as well as the coagulation efficiency of microfiltration with in-line rapid pre-coagulation using an instantaneous flash mixer. © 2005 Elsevier B.V. All rights reserved.