In this study, an ultrafiltration process was used for olive oil mill wastewater pretreatment. Chitosan was used to increase the efficiency of the ultrafiltration process. The Box-Behnken statistical exper-iment design method was used to determine the effects of operating parameters such as chitosan concentration, feed flowrate and ultrafiltration time on permeate flux and percent chemical oxygen demand (COD) removal. The response function coefficients were calculated using the Design-Expert 7.0 program. The predicted values of permeate flux and COD removal efficiency obtained using the response function were in good agreement with the experimental data. The permeate flux increased with the increase in flow rate and decreased with the increase in chitosan concentration. The decline in flux was not significant during the whole filtration period. So, the optimum set of chitosan concentra-tion and feed flowrate for permeate flux was 100 mg/L and 200 L/h, respectively, with 43 L/m2 center dot h flux value at 30 min operation time. On the other hand, COD removal efficiency decreased with increas-ing flow rate and increased with increasing chitosan concentration. The optimal set was 600 mg/L and 100 L/h for COD removal, with 80% COD removal efficiency at 80 min of ultrafiltration time.