Despite various studies on heat transfer by natural convection between long smooth vertical parallel plates heated at constant temperature or constant heat flux, no information is available on heat transfer by complicated natural convection between long heated plates accompanied by acceleration or deceleration downstream. Here, such heat transfer was considered experimentally and numerically. Heat transfer characteristics were investigated by generating natural convection between long smooth heated plates of which the channel cross-sectional area was changed in the downstream direction. At the same time, a mathematical model based on the physical phenomenon was built, and the experimental results and numerical computation were compared. As a result, it was shown that heat transfer by natural convection changes locally in accordance with the channel cross-sectional area, and that a unique phenomenon occurs that cannot be predicted from conventional knowledge of heat transfer by natural convection.