Experimental investigations were carried out regarding natural convection heat transfer of Nanoparticle Enhanced Ionic Liquids (NEILs) in rectangular enclosures of two different sizes with dimensions length × width × height, 50 × 50 × 50 mm and 50 × 50 × 75 mm in heated from below. The NEILs were synthesized by dispersing different wt% (0.5, 1.0, and 2.5) of aluminum oxide (Al₂O₃) nanoparticles of two different particle shapes (spherical and whiskers) into N-butyl-N-methylpyrrolidinium bis{(trifluoromethyl)sulfonyl} imide, ([C₄mpyrr][NTf₂]) ionic liquid (IL). Heat transfer related thermophysical properties, i.e. density, viscosity, thermal conductivity, and heat capacity of base IL and NEILs were also measured and reported. The experimental measurement shows enhanced density, thermal conductivity, viscosity, and heat capacity of NEILs compared to the base IL and they increase with the nanoparticle concentration. However natural convection heat transfer coefficient was observed to deteriorate for the NEILs compared to the base IL irrespective of the shapes of the particles and aspect ratio of the enclosure and the deterioration increases with the increase of nanoparticle concentration. Interestingly spherical Al₂O₃ NEILs was observed to affect more adversely compared to the whiskers Al₂O₃ NEILs. The observed degradation of the heat transfer performance of the NEILs could not fully be explained by the change of thermophysical properties, which indicates that other factors may play significant roles in this phenomenon and the possible reasons of the degradation is discussed in this paper.