To understand the physical processes of saline intrusion in Red river and Thai Binh river systems, we carried out numerical simulations using one-dimensional model, MIKE 11. The data for calibration and validation are observed in local stations and the recent field data measured in nine tributaries. We found significant differences in the physical process of the saline intrusion between the two river systems owing to the tidal phases, river discharges and geometrical characteristics of the river mouth.