This study introduces Fraxinus tree leaves as a new, efficient biosorbent of As(III). A suitable response surface was achieved by running a central composite design. Simultaneous optimization of both responses (R% and q) was carried out and 67% of the goal of desirability function was attained. The results obtained for simultaneous optimization are R = 70% and q = 80.6mg g(-1) with 67% desirability in m = 600mg L-1 where s = 0.10 g and pH = 3.9. Langmuir and Freundlich isotherms model were applied in explaining the sorbent-sorbate equilibrium study, and maximum capacity uptake equals 99.97 mg g(-1) and K-L = 0.05 L mg(-1) has been obtained. Fourier Transfer Infra-Red (FT-IR) and kinetic results were considered to examine the functional groups involved and the adsorption mechanism.