The aim of the present study is to develop a mathematical model for a better understanding and prediction of the Ultrafiltration membrane fouling, which can be applied to the heterogeneous membrane with non-uniform pore size. Polysulfone flat membrane with a molecular weight cut-off (MWCO) of 60000 (nominal size of 0.05 mu m) was used with a constant feed flow rate and a cross-flow mode in ultrafiltration of a latex paint solution. The pore size distribution of the heterogeneous membrane was estimated using ImageJ software. The model was developed based on analyses of particle attachments. All possible depositional and coagulation fouling attachments were included in the model with the evidence derived from the SEM images. Monodisperse particles with sizes of 50nm and 100nm, as well as the latex effluent with a wide range of particle size distribution, were utilized in order to test the validity of the model. The transmembrane pressure estimated from the mathematical model agreed with the transmembrane pressure experimentally measured mostly within 3.3-10.0% error and up to 13.0% error, using the monodisperse particles and latex effluent, respectively.