A rapid method using the pilot in-line filter to detect any change in coagulation performance was a proposed in this study. This method attempted to detect a change in coagulant dosage and mixing intensity by evaluating the filtrate quality of the in-line filter, which took the rapidly mixed water. Since the response time of this method was less than 10 min, it could be valuable to monitor the coagulation performance. The in-line filter was found more useful without underdrain. The in-line filter was more sensitive to a change in filtrate quality without underdrain than with underdrain. A new method, which combines a jar test with the in-line filter, was proposed to determine the coagulant dosage. This method reflected the actual plant situation more accurately than a jar test.