A numerical investigation through laminar natural convection has been executed to illuminate the effect of curvature ratio in an arch enclosure filled with water. The left side wall of the cavity is maintained at a higher temperature than that of the right side wall while the other walls are kept insulated. The governing equations such as continuity, momentum and energy equation are solved by finite volume method. The effect of pertinent parameters such as curvature ratio (1≤ CR ≤ 1.5) and Rayleigh number (1×10⁴ ≤ Ra ≤ 1×10⁶) and) on heat transfer are calculated by commercial available computational fluid dynamics software, FLUENT 6.3. The fluid flow and heat transfer are shown by streamlines, isotherms and velocity contour. From the investigation, it has been reported that the varying height of the arch cavity and Rayleigh number playing a significant role in heat transfer rate. The variation in heat transfer is calculated in terms of average Nusselt number.