clouds or stochastic spirals, gravitational interaction with dark halo subunits, major and minor mergers (see Jenkins, Binney (1990); Binney, Tremaine (2008); Shapiro et al. (2003)). As far as the velocity dispersion of stars increases the efficiency of all mechanisms decreases. For initially dynamically cold disk, another mechanism capable of effectively heating a disk is the development of gravitational instability against random perturbations. It leads to the increase of stellar velocity dispersion both in the plane of a disk, and along the vertical axis (through the developing of bending instability). At the early stage of evolution during the disk mass growth, stellar velocity dispersion might remain fitted to the level of its marginal stability. If a total disk mass is constant and there exists a gas disk, forming stars with low velocity dispersion in its plane, a self-regulating process, supporting stellar disk velocity dispersion in the quasi-stationary station, is also possible (Betrin, Lin, 1987). In any case, even if gas and young stars are absent, the minimal velocity dispersion of old stars at given $R$ will be constrained by disk local marginal stability condition. Numerical simulations of models of initially weakly unstable 3D disks show the fast passage of a disk 