Fluidized beds are nonlinear dynamic systems that exchange mass and energy with outside. They are governed not only by fluid dynamics, but by thermodynamics, especially the second law of thermodynamics as well. According to Prigogine's dissipative structure theory, the following have been concluded: (1) a fixed bed is on thermodynamic blanch, and bubbling, turbulent and fast beds are on the dissipatve structure branches. (2) Entropy in fluidized beds is divided into two parts: entropy production and entropy flux. The latter increases with gas velocity and decreases with pressure of the systems. That means the entropy of a system may reduce and the system with higher gas velocity behaves as dissipative structure characteristics. (3) For a given velocity, a fluidized bed operates stably on the whole, but it is unstable to local gas-solid phases. The unstable phases are described by fluid dynamic equations, While the minimum of system energy function assures whole stability of the system. (4) A transition criterion of a bubbling bed is derived from Prigogine's stability theory.