The sorption of metal ions from sodium arsenate solution by using an agricultural by-product, tree fern, has been investigated. The sorption of sodium cation on tree fern has been studied in terms of pseudo-second-order sorption processes. The pseudo second-order kinetic model has been developed to predict the rate constant of sorption, the equilibrium capacity and initial sorption rate with the effect of initial metal ion concentration. The Henry's law, Langmuir and Redlich-Peterson isotherms were used to correlate experimental isotherms. The sorption of sodium cation follows Henry's law and a modified Redlich-Peterson isotherm. However, the arsenic could not be removed from sodium arsenate solution by the sorption for the reason that arsenic is in the form of anion as arsenic acids in the solution and because of the negative charge of tree fern surface.