Pen shells which have some functional groups like amide, amino and carboxyl groups were used as an adsorbent to remove cadmium ions from an aqueous solution. The functional groups in pen shells were investigated by FT-IR and the surface condition of pen shells and existence of cadmium ions onto the pen shells were confirmed by the SEM and EDX analyses. The highest adsorption capacity could be achieved as about 35.652 mg/g at the initial pH 4 of aqueous solution. Two adsorption isotherm models namely, Langmuir and Freundlich were used to fit the equilibrium data and the experimental data well follows the Langmuir isotherm model (r2 = 0.992) and the maximum adsorption capacity (qm) for cadmium ions was achieved as the 37.630 mg/g by the Langmuir model. Also, adsorption kinetics for cadmium on the pen shells followed pseudo second order rate equation with 0.999 of correlation coefficient and the calculated qe value (35.594 mg/g) was in excellent agreement with the experimental qe value (37.630 mg/g). Changing of temperature of the aqueous solution could not greatly affect during the adsorption process between cadmium ions and pen shells. Also, pen shells had a high selectivity to the cadmium ions In addition, the desorption efficiency of cadmium ions was about 98.0% at the 0.5 M of HCl and adsorption capacity of cadmium ions for reused pen shells can be still maintained at 31.65 mg/g level until the 2nd cycle. From the result, adsorption process using pen shells could be applied to the removal system of cadmium ions from aqueous solution and it was suggested that conventional treatment process using commercial ion-exchange resin can be sufficiently replaced with economical process using waste pen shells with comparative high adsorption capacity.