Environmental pollution is an important global issue. In this study, adsorption of Ni (II) ions from aqueous solution was evaluated. Ni (II) was selected as an adsorbent due to its toxic properties, and optimum adsorption pH was determined to be 5. Langmuir adsorption isotherm model was used to determine the maximum adsorption capacities of activated sludge (95. 88 and 78 mg g(-1) at 20, 35 and 50 degrees C, respectively. Initial metal concentrations and particle size of activated sludge affected its adsorption capacity, which increased with increasing, initial metal concentrations, but decreased with increasing particle size of adsorbent. Surface structure and functional groups of activated sludge biomass were determined by FT-IR and SEM analyses. Different functional groups have been identified for activated sludge. These functional groups can react with metal ions in aqueous solution. The pseudo second-order and intra-particle diffusion kinetic models were used to determine adsorption kinetics. Both kinetic models well described the adsorption of Ni (II).