The genesis of Mei-Yu fronts is studied by assuming the fronts are characterized by a line of low-level potential vorticity (PV) anomaly. It is proposed that the frontogenetic process is initiated by the CISK mechanism through the interactions between the PV anomaly and the convection induced by Ekman layer pumping. The scale contraction produced by the convergence flow of the convection provides the basic frontogenetic forcing. In addition, the cross-frontal horizontal scale of the front contracts at a rate faster than what can be expected from a linear theory, due to the presence of a nonlinear feedback process. The process is due to the fact that the effect of condensation on PV is proportional to the value of absolute vorticity. These processes are illustrated using both a linear analysis and a nonlinear numerical model.