Effects of an obstacle placed over a heat source at the bottom surface of an enclosure on natural convection have been studied numerically. Computations were conducted for various widths of an obstacle with an aspect ratio of the enclosure of 1 and for various aspect ratios with a constant value of width of an obstacle. The results show that flow patterns can be roughly divided into three types: (1) fluid flows mainly over the obstacle, (2) fluid flows both over and through under the obstacle, and (3) flow is limited under the obstacle. When the flow is limited in the region over the obstacle, heat transfer is hindered. When the fluid flows through under the obstacle, however, heat transfer reduction is diminished.