This paper deals with a theoretical analysis of heat transfer from an inclined thin flat plate by natural convection, by using same analytical method as the author treated for the heat transfer from a horizontal thin flat plate. If the laminar boundary layer exists all around the plate, the local heat transfer coefficient increases at a place near the lowest point of plate, and the difference of the coefficient between the upper and lower surface decreases as the inclined angle from the horizontal plane is increased. The average heat transfer coefficient of the lower surface is larger than that of the upper surface, and the average coefficient increases as the plate approaches to the vertical position.