- The evolution of the isolated, axially Jeans-stable (an initial velocity dispersion of particles is equal to Toomre's stabilizing one), and rapidly rotating stellar disk of flat galaxies is investigated by N-body simulations. An algorithm of the direct summation for a simulation code is used. As the model evolves during the first rotation, the density wave spiral structure develops in the plane. At somewhat later times, in the normal to the plane direction, a characteristic warping of the outer disk begins to grow. The origin of this new phenomenon, which has not been illustrated previously via simulations, is studied. It seems likely that the outer vertical resonance produces the warp.