The purpose of this research was to investigate the removal of chromium (VI) from an aqueous solution using the waste of bio-adsorbents, oil cake, by design of experiment and modeling via response surface methodology. The research process was conducted in two stages, taking into account different variables, including studied pH, temperature, absorber dosage, initial concentration of chromium ion and mixing speed. By response surface methodology, three-dimensional graphs were plotted which showed the effects and interactions of variables. Increasing on temperature from 30°C to 40°C caused more absorption of chromium; however more increase, to 50°C, led to a decrease in the absorption. At lower pH, higher adsorption occurred. In the more acidic environment, the changes in the temperatures were more effective. In the second stage, the resulting three-dimensional graphs showed pages with positive slope, versus three variables of absorber dosage, initial concentration of chromium ion and mixing speed. Increasing the dosage, initial concentration and speed, had great effects on the absorption of chromium. Equations obtained from response surface methodology represented the mathematical relationship between the absorption of chromium as a function of variables; with coefficients of determination above 95%. Absorption values for different circumstances were predicted using the model equations and compared with the experimental results, which showed the desirable fittings.