The effect of coagulant dosage on the filterability of coagulated humic acid solutions was studied by dosing polyaluminum chloride (PAC) as a coagulant. The experimental results showed that the conventional cake filtration theory (known as Ruh's filtration equation) can be applied to dead-end filtration analysis of dissolved humic acid solutions and/or coagulated humic acid suspensions. Increasing dosage of coagulant resulted in a decrease in the average specific filtration resistance of cake, and an optimum dosage was observed. For dissolved humic acid solutions, the average specific filtration resistance decreased significantly with an increase in solute concentration. The filterability of humic acid cake could be classified into three different regions, depending on humic acid concentration. Experimental results for coagulated humic acid suspensions coincided favorably with the calculated results based on the compressible cake filtration theory.