The ducting mechanism of acoustic-gravity waves in a thermally stratified atmosphere with spherical geometry is investigated analytically. By introducing a reasonable atmospheric sound speed model, the Green's function of Navier-Stokes equations is derived. The dispersion relation of ducted acoustic-gravity wave modes is studied. Calculations reported here show various ducted modes, namely, the quasi-surface modes, the fully ducted modes, and the imperfectly ducted mode. The ducting mechanism of the leaky gravity wave mode, usually referred to as mesospherically ducted mode, is reexamined. We find that the sudden rise of temperature at the lower thermosphere can itself contribute to the ducting of the lower atmospheric gravity waves. ¿ 1998 American Geophysical Union