In recent times, convective heat transfer using nanofluid has been a active field of study. However experimental studies pertaining to buoyancy induced convective heat transfer using various nanofluid is relatively scarce. In present study, a square enclosure of dimensions (40 x 40 x 200) mm is used as test section. Initially, Al2 O3 /Water nanofluid with volume fractions 0.3%, 1% and 2% and Rayleigh numbers ranging from 7 x 105 to 1 x 107 are studied. These results are then compared with Ho's[1] experimental data. Nusselt number is calculated based on the thermo-physical properties that are measured in-house for the given conditions. Further, MWCNT/Water nanofluid with volume fractions 0.1%, 0.3% and 0.5% is formulated and are studied for various Rayleigh numbers. Comparison of Al2O3 /Water and MWCNT/Water nanofluid have been made for different volume fractions and for various range of Rayleigh numbers. It is observed that MWCNT/Water nanofluid when compared with Al2 O3 /Water nanofluid yields higher values of the Nusselt number for a given volume fractions. All the existing experimental studies using particle based nanofluid concluded a deterioration in natural convective heat transfer. This study for the first time demonstrates an enhancement in natural convection using MWCNT/Water nanofluid. Such enhancement cannot be simply explained based only on the relative changes in the thermophysical properties. Other factors such as percolation network in MWCNT/Water nanofluid which increases the heat transfer pathway between two walls and the role of slip mechanisms might be the possible reasons for the enhancement. © Published under licence by IOP Publishing Ltd.