Natural convection heat transfer in a vertical trapezoidal enclosure was investigated experimentally. Two opposite parallel surfaces were kept in a vertical position and were maintained at uniform temperature, while the floor and the ceiling were insulated. The experiments were carried out using water in the range of 2.0×107<Raw<2.0×108 for the cases of horizontal upper-surface and horizontal lower-surface. The ratios of the cold surface length to the hot surface length were varied in the range of 0.41.6. The paper presents the method of evaluating the temperature distribution, and the average Nusselt number of the natural convection in the trapezoidal enclosure by refering to the temperature distribution and the Nusselt number in the rectangular enclosure.