The three dimensional effect of natural convection on a miniscale isothermal plate in an enclosure is numerically studied. The 3D effect of the natural convection increases with scale reduction, and consequently the heat transfer is enhanced. For a given plate, the heat transfer features are different for the plate positioned vertically or horizontally. If the plate scale is relatively large, the heat transfer in the vertically positioned case is stronger than that in the horizontally positioned case, arid the difference between the both extents of heat transfer becomes smaller as the scale decreases.