Flux decline in crossflow ultrafiltration and microfiltration was investigated by perceiving membrane fouling as a dynamic process from non-equilibrium to equilibrium. Under the influence of the boundary condition at the initial section and cross flow, the equilibrium is first reached at the initial section of the crossflow filter and the front of the equilibrium region progresses with time toward the end of the filter. A mathematical model was developed to describe this dynamic process and a closed-form solution of the model was provided. With the model, the time-dependent flux and the time required to reach the steady state in a crossflow filtration can be easily determined. The fouling process under different conditions is simulated as an illustration and demonstration of the newly developed model.