Flux decline during stirred UF over a long time of operation can be classified into short and long time domains. In the short time, the decline in flux is basically osmotic pressure limited and this decline is extremely rapid. The limiting flux for the osmotic pressure controlled domain is obtained by successive substitution technique. This technique includes the variations of properties with concentrations. A modified Sherwood number coorelation for determining the mass transfer coefficient is proposed for laminar flow in a stirred cell. The long term flux decline can be modeled semi-empirically by a two parameter model. One of these, the steady state polarized layer resistance, can be correlated to the operating conditions while the other constant of the model is found to be a characteristic of the solute only. The simplistic model developed here to account the flux decline during UF may be useful to the process engineers for design purposes.