An experimental study on natural convection heat transfer in two vertical parallel plates with a closed lower end was performed in air using isothermal plates with heights of 100 and 200 mm, and the results were compared with those of numerical analysis. The Grashof number range in the experiment was about 106 to 108 and the ratio of hot and cold wall temperatures was from 0.4 to 1. In the dimensionless distance of the parallel plates of from 0.1 to 0.3, the temperature of the air between the plates fluctuated considerably over time. The heat transfer coefficients of the plates in the experiment exhibit good agreement with the numerical results when GrsS2 >__- 106, but when GrsS2 < 106, they are slightly smaller than the numerical results, at R=1.