Variations of the filtration rate with time and the properties of the filter cake were examined for microfiltration of binary mixtures of fine particles. Both the average porosity and the average specific filtration resistance were determined from filtration experiments in which a filter was subjected to a sudden reduction in its filtration area. The average porosity in the filter cake of the binary mixtures of fine particles was explained well by the mixture packing model in which small particles fill the voids between large particles. The decrease in the ratio r of small to large particle size caused a significant increase in the value of beta, which is a measure of the mixing action between large and small particles. In addition, the average specific filtration resistance of the binary mixtures was obtained on the basis of the additive law of the effective specific surface area. The filtration rate in the binary mixtures was evaluated accurately by using both the average porosity and the average specific filtration resistance.