In this research, validity of temperature-independent thermophysical properties assumption of water-Al203 nanofluid in natural convection problems within the enclosures is investigated. The numerical results are obtained utilizing an in-house finite volume code based on the SIMPLE algorithm. In order to do the validation the numerical results and those of existing correlations are compared. In order to evaluate the thermal performance of the enclosure, the average Nusselt number on the hot side wall in both temperature-independent and dependent cases is compared Results show that, in the all considered solid volume fractions, the difference in the Nusselt number in the case of temperature-independent properties is less than 10 percent in comparison with the case in which the properties are temperature-dependent when temperature difference is less than 5 ○C. As the temperature increases, the difference between Nusselt number in both cases increases and the effect of increase in solid volume fraction is to increase this difference. Results also show that the difference between these two cases is dependent solely on temperature differences between the hot and cold walls regardless of the temperature they have.