Natural convection heat transfer in a partially divided trapezoidal enclosure is studied numerically using the control volume method. Summer and winter conditions are separately examined by imposing regarding thermal boundary conditions. A horizontal divider included and its two different placements are considered. It is shown that heat transfer results are not significantly altered by the presence of the divider for summer condition. For winter condition, on the other hand, decrement in heat loss and effect on the flow and the temperature fields by the presence and the placement of the divider are observed, respectively. As a horizontal divider is placed to oppose buoyancy, the flow strength becomes weaker and formation of two separate levels of temperature uniformity occurs. As the divider is placed to assist buoyancy, the flow gets stronger and tends to form relatively uniform temperature field within whole enclosure.