This study would aim to investigate the biosorption of mercury. A number of isotherm models were analyzed for experimental data by cystoseira myricaas as brown algae for two biomass sizes (50 µm and 300 µm). In addition, several kinetic models have studied for two initial concentrations of mercury (100 mg/L and 150 mg/L). Furthermore, the thermodynamic values (?G0, ?H0 and ?S0) were investigated in different algae sizes (75 µm, 200 µm and 1000 µm). The best conditions containing pH, temperature, sorbent dosage, metal concentration and contact time were found by response surface methodology (RSM) and Fuzzy modeling. The maximum removal of mercury was predicted 93.7% and 95.20%, by RSM and Fuzzy modeling respectively.