Natural convection heat transfer within a rectangular enclosure provided with a partition extended from the vertical heated wall was investigated experimentally. The experiments were carried out with water, for Rayleigh number in the range, 2.0×107 < Raw < 3.0 × 108, and an aspect ratio H/W=s. The effect of partition on the fluid and temperature fields was investigated by dye-injection flow visualization and by thermocouple probes, respectively. The effect of the partition on the heat transfer across the enclosure was also studied. The slant partition, especially, the downward oriented partition reduces the convective heat transfer in comparison with that of horizontal partition. And only the horizontally projected length of partition, independently of the angle of inclination and the vertically projected length of the slant partition, effects on the Nusselt number Nuw distinctly.