Abstarct: In the present study, walnut shell powder (WNS), a low cost locally available as an adsorbent was used for the removal of brilliant green (BG) dye from aqueous solutions. The shell were ground, sieved to a defined size range. The shells were first activated by 1.00 M of NaOH for 1.00 day.The prepared adsorbent was characterized by surface area measurment, Fourier transform infrared (FTIR) spectroscopy and scanning electron microscope (SEM). Effect of various physical and chemical parameters such as pH, adsorbent dose, contact time, initial dye concentration, size of particle and temperature were investigated. At optimum values of the above mentioned parameters, more than 95% removal efficiency was obtained within 10.0 min at adsorbent dose of 0.0400 g/100 ml for initial dye concentration of 50.0 mg/L at pH=7. The adsorption of dye was found to follow a pseudo-second-order rate equation. Analysis of adsorption results obtained at different temperatures showed that the adsorption pattern on the adsorbent can be described perfectly with Freundlich isotherm models, and the characteristic parameters for adsorption isotherm were also determined. Free energy of adsorption (ΔG0 ), enthalpy (ΔH0) and entropy (ΔS0) changes are calculated to know the nature of adsorption. The calculated values of ΔG0 indicate that the adsorption process is spontaneous. The estimated values of ΔH0 and ΔS0 both show the positive sign, which indicate that the adsorption process is endothermic and the dye molecules are organized on the adsorbent surface in more randomly fashion than in solution.