Activated carbons were prepared from agricultural wastes by chemical activation with KOH. The seed of water-melon was used as raw materials. Preparation process involves the roasting of raw material and carbonization of roasted material followed by chemical activation. N₂-BET surface areas and total pore volumes of activated water-melon char prepared by chemical activation were measured as 128 ~ 978 m²/g and 0.002 ~ 0.317 cm³/g, respectively. Adsorption of methylene blue in solution by activated carbon was carried out and the adsorbate transmittance of methylene blue was determined UV absorption at 660 nm using UV spectrophotometer.