In this study, the usage of abundantly available oil palm mesocarp fiber as biosorbent for nickel removal from aqueous solution is discussed. The effect of NaOH and HCl pretreatment on the nickel removal by oil palm mesocarp fiber is one of the objectives. Varieties of NaOH concentration were examined to determine the effect of concentration on nickel removal. Other than pretreatment, effect of initial nickel concentration is also determined. XRD analysis was done in order to proof the existence of nickel phase on the oil palm mesocarp fiber. From the result, oil palm mesocarp fiber treated with NaOH results in higher nickel adsorption compared with the treatment using HCl and the nickel nitrate phases was successfully attached on the oil palm mesocarp fiber with crystalline size between 28-52 nm.