An Increase in jet penetration due to the wall boundary layer is determined in the flow field including an aerodynamic interference between the wall boundary layer and the jet. The aerodynamic effect of the wall boundary layer is replaced by that of a secondary vortex filament resulting from vorticity in the wall boundary layer. A differential equation governing the increase in jet penetration is derived using the circulation around the secondary vortex filaments, its induced velocity and the empirical decay law of the jet axial velocity along the jet centerline. The circulation around the secondary vortex filament is estimated according to Hawthorne's theory (1)* and expressed in terms of aerodynamic characteristics of the wall boundary layer. A numerical example of the present analysis shows a fairly good agreement with the experiment. This indicates that the used vortex flow model simulates the real flow conditions well.