The aim of this research is to design a rainwater filtration system by using an activated carbon. The activated carbon had been act as one of the filter medium to filtrate the water. Three filters were design, namely Filter A, Filter B and Filter C, to determine the most effective filter that can be used in the study. To determine the effectiveness of the filter, seven parameters were measured that is temperature, colour, turbidity, total suspended solid (TSS), pH, conductivity and Biochemical Oxygen Demand (BOD 5). The results shows that the removal of BOD 5 is up to 82%, TSS removal is 55.6% and turbidity removal is 28.8%.