In our recent study of the equilibrium configuration of the near-Earth magnetotail, it is found that a steeper profile of entropy function S leads to the formation of a thinner current sheet. Here S=pV&ggr;, where p is the plasma pressure and V is the volume of a unit magnetic flux tube. In the presence of density gradient at the edges of the current sheet, the lower-hybrid-drift instability may lead to pressure diffusion. Since the pressure generally decreases and the entropy function S increases tailward, the diffusion of plasma pressure results in a steepening, or antidiffusion, of the entropy profile. This, in turn, leads to a thinner current sheet, which enhances the pressure diffusion. This positive feedback process is called the entropy antidiffusion instability. On the basis of MHD simulations with a pressure diffusion, we find that the entropy antidiffusion instability leads to a further thinning of the near-Earth current sheet and the onset of dipolarization of near-Earth geomagnetic field lines. The growth rate of the instability is linearly proportional to the diffusion coefficient. The growth time at the final stage for the formation of a very thin current sheet is ~1 min. This instability may explain the observed explosive growth phase of substorms and the onset of dipolarization of geomagnetic field lines. ¿ 1998 American Geophysical Union