Abstract Two-dimensional, steady, laminar natural convection in a partially open cavity with a divider is studied numerically. The cavity consists of two horizontal adiabatic walls, a vertical hot wall and a vertical cold wall with an opening. A thin-poorly conducting divider projects from the bottom adiabatic wall. Flow and heat transfer in the divided, partially-open cavity are examined for different Rayleigh number, Ra, divider height, H, and opening size, Y o . Results show that the presence of the divider significantly influences the flow and heat transfer in the partially-open cavity. The influence of both the divider height and the opening size becomes stronger at larger Rayleigh number. For fixed Ra and H, as Y o increases, the average Nusselt number of the hot wall and the maximum stream function remain nearly unchanged when Y o is smaller than H, but increase significantl y when Y o is larger than H. Ten typical flow patterns in the partially-open cavity with a divider are found. The streamlines and isotherms, variation of local and average Nusselt numbers over the hot and cold walls, the features of the opening, provide valuable insight into the physical processes.