A mathematical model is developed for quantification of concentration polarization phenomena during osmotic pressure controlled ultrafiltration under turbulent flow conditions. An integral method is used for the solution of the concentration profile in the developing mass transfer boundary layer in a rectangular channel. The model is used to predict the permeate flux and observed retention simultaneously during cross flow ultrafiltration. The results from the numerical solution of the model equations are compared with the existing data from the literature for dextran (T-70) over a range of operating conditions. The predictions are found to be in excellent agreement with the experimental results. Furthermore, a detailed parametric study has been undertaken to investigate the effects of system parameters (e.g. pressure difference, cross flow velocity, bulk concentration, real retention, etc.) on the system performance (permeate flux and observed retention).