The bioaccumulation of UO22+ (uranyl ion) was investigated by using dry biomass of Botryococcus braunii. Experiments were conducted based on a rotatable central composite design (CCD) and analyzed using response surface methodology (RSM), which was used to optimize the bioaccumulation process. In this goal, the effects of parameters such as initial uranium (VI) concentration, pH, temperature and contact time were studied. The optimum conditions for maximum removal (87%) of UO22+ from an aqueous solution were as follows: initial uranium (VI) concentration (55 mg/l), pH (3.6), temperature (46 degrees C) and contact time (74 min). Langmuir and Freundlich isotherms and also thermodynamic parameters like Enthalpy (Delta H degrees), Entropy (Delta S degrees), standard Gibbs free energy (Delta G degrees) were investigated, as well.