The purpose of this study was to develop a dynamic model to adequately describe an Upflow Anaerobic Sludge Blanket (UASB) reactor. To achieve the aim some available models of a UASB reactor were studied in order to modify their drawbacks and propose a new improved model with less complexity. The developed model was based on a model proposed by Pontes et. al[1] in 2006 which was composed of a kinetic model with integration of a hydraulic model. According to this model, a UASB rector was divided into three compartments and each compartment was assumed to behave as an ideal reactor. Following terms was of importance in the developed model: degradation of substrate which is followed by a kinetic model, the non-idealities of a UASB reactor caused by channeling, dispersion and stagnant points as well as the biomass transportation between two compartments. To investigate the validity of the model two simulations were performed. According to the simulation results, the developed model was in good agreement with experimental data. Moreover, through a sensitivity analysis it was concluded that the model was sensitive to variation of kinetic parameters.