Waste cooking oil can be considered as an alternative biodiesel feedstock for replacing edible oils. However, this feedstock can not be directly used since it contains much impurities and high Free Fatty Acid (FFA) content. Thus, pre-treatment process is required to enhance the feedstock quality. Adsorption using activated carbon is one of various methods that can be applied to reduce FFA content which is relatively easy and cheap. Coconut fiber is biomass waste that can be utilized in activated carbon production. This work has successfully synthesized activated carbon from coconut fiber with activator medium of H3PO4 10% weight and carbonization temperature of 600 °C, indicated from yield, water content, ash content, and methylene blue adsorption capacity. The yield of carbonization process developed in this work reached 40% while the yield for water content, ash content, and methylene blue adsorption capacity were 2.5%, 2.3% and 1646.1 mg/g carbon, respectively which complied with SNI 06-3730-1995.  This adsorbent was tested on fixed bed adsorption column with FFA reduction reached up to 93% at waste cooking oil flowrate of 3 ml/min for 45 minutes operation time.