Ulmus carpinifolia tree leaves were successfully used to remove Tl(I) from aqueous solution in a batch system. In order to improve the uptake capacity of sorbent, it was modified by various chemical agents such as NaOH, HNO3, NH3, NaCl, NaHCO3, and CaCl2. Among the modifiers, NaCl was the best. Equilibrium behavior of sorbent with Tl(I) was examined by the several isotherms. Considering modified U. carpinifolia equilibrium data fitted well to the Langmuir model with maximum capacity of 54.6 mg/g. The other isotherms such as: Freundlich and Dubinin-Redushkevich (D-R) models were also examined. The central composite design (CCD) was successfully employed for optimization of biosorption process. An empirical model was given through using response surface methodology. Also its validation was recognized by using relevant statistical tests such as ANOVA. The optimum conditions of biosorption: pH, m (amount of sorbent) and C (initial concentration) were found to be 7.9, 11.4 g/L, and 8.8 mg/L, respectively. On the other hand thermodynamic parameters: ΔG, ΔH, and ΔS were evaluated: the obtained results show that biosorption process was spontaneous and exothermic. Eventually, FT-IR analysis confirmed that the main functional groups of sorbent have been involved through the biosorption process.