The natural convection heat transfer of a differentially heated cavity filled with porous material and saturated with nanofluid is studied. The nanofluid used in the present study contains 60% Ethylene glycol, 40% DI-water and 30 nm size Al2O3 nanoparticles. The volume concentration of nanofluid used is in the range of 0.05% ⩽ ϕ ⩽ 0.4%. The range of Rayleigh number in the present study is 1.2 × 108 ⩽ Ra ⩽ 4 × 108 for clear cavity and 3 × 103 ⩽ Ra ⩽ 1.3 × 104 for the porous cavity. Viscosity of the nanofluid is also measured at volume concentration of 0.05% and found one available model works for the calculations. In order to explain the heat transfer behaviour of the present system, heat transferred by both clear and porous cavity, heat transfer coefficients of both hot and cold wall, as well as Nusselt number variation with concentrations of nanofluids are presented. It is found that the performance of porous cavity filled with a nanofluid volume concentration of 0.05% is enhanced while the other concentrations of nanofluids deteriorate the performance. At a volume concentration of 0.05%, the heat transfer capability of porous cavity is enhanced to a maximum of 10% compared to the base fluids.