For predicting fresh-salt water interface in unconfined coastal aquifers, an equation has been developed to estimate the advance of interface which depends on; the densities of fresh and salt water, the hydraulic properties of the aquifer and the flow velocity components at the boundaries of the aquifer. The developed equation is tried in Sidi Kreir Locality, 32 km west of Alexandria (north-western coastal zone), constituting a coastal unconfined aquifer. The values for advance of interface estimated by the developed equation are compared with those computed using Ghyben-Herzberg relation and are found to have an order of magnitude agreement and are of nearer values. The developed equation is considered advantageous because it considers the hydraulic properties of the aquifer as well as the laws governing the potential flow of two liquids in the porous medium. Assessment of the goodness of the fitting of the compared values for advance of interface shows that; the analytical solution developed, seems to be successful procedure for predicting the movement of the fresh-salt water interface in coastal unconfined aquifers existing in favorable areas