Water hyacinth, an aquatic plant in Lake Tana, was used as a raw material for the synthesis of activated carbon. It was prepared by chemical activation using phosphoric acid followed by carbonization at 4000C for two hours. The activated carbon has been used for the removal of methylene blue (MB) dye. Fourier Transform Infrared Spectroscopy (FT-IR) was carried out in order to identify the functional groups present on the activated carbon, the surface area was 163.29 m2/g and different physico-chemical properties were determined. Effects of different parameters like solution pH, temperature, adsorbent dosage, contact time and initial dye concentration were studied under batch adsorption. The concentration of methylene blue dye was determined by UV-Vis spectroscopy instrument. The maximum removal of dye (98.6%) was achieved at pH 10, 1.5g adsorbent dosage and 60 min at a dye concentration of 15 mg/L. The adsorption isotherms fitted well to both the Langmuir and Freundlich equations. Adsorption of MB on WHAC (R2 = 0.97) followed Langmuir model and in Freundlich isotherm model (R2 = 0.9907). The pseudo second order kinetic model provided a better fit for the adsorption data.