This study aims to investigate the adsorption of C.I. Acid Red 97 (AR97) onto walnut shell (WS) in order to find out the desired conditions. The effect of operational parameters including adsorbent dosage, agitation speed, contact time, pH and temperature was studied in a batch mode. The results revealed that 73.33 % of AR97 (50 mg/L) was decolorized at 60 min of contact time, agitation rate of 140 rpm and 12 g/L of adsorbent at 291 K. The rate of adsorption follows pseudo-second-order kinetics with the high correlation coefficient, which was more than 0.99. Specific surface area of WS was calculated as 1.28 m2/g by BET method. It was observed from the values of thermodynamics parameters such as Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) that the nature of adsorption process is spontaneous, exothermic and mainly physical. Langmuir, Fruendlich and Redlich–Peterson isotherms were applied for mathematical description of equilibrium data and findings indicated that it is in good agreement with Fruendlich model.