Heat transfer by natural convection that develops between heated plates accompanied by acceleration or deceleration in the downstream direction was examined experimentally and numerically. This phenomenon occurs in various environments, for example, in the aeration layer that accompanies hybrid external thermal insulation of buildings and in cooling equipment of large electronic devices, and it is important, therefore, to solve the thermo-hydrodynamic characteristics of natural convection induced therein. Despite numerous studies on heat transfer by natural convection that develops between smooth vertical parallel plates when plate temperature or heat flux is constant, no information is available on the heat transfer by complicated natural convection that develops between heated plates accompanied by acceleration or deceleration in the downstream direction. Here, heat transfer characteristics were investigated by generating natural convection between smooth heated plates between which the channel cross-section area was changed in the downstream direction. A mathematical model based on the physical phenomenon was built simultaneously, and the experimental results and the numerical computation were compared. It was found that the heat transfer by natural convection changes locally in accordance with the channel cross-section area, and that a unique phenomenon occurs that is not explainable by conventional knowledge of heat transfer by natural convection.