Effect of copper–water nanofluid as a cooling medium has been studied to simulate the behavior of heat transfer due to laminar natural convection in a differentially heated square cavity. The transport equations for a non-Newtonian fluid have been solved numerically following finite volume approach using SIMPLER algorithm. The shear stresses have calculated using Ostwald–de Waele model for an incompressible non-Newtonian fluid. The thermal conductivity of the nanofluid has been calculated from the proposed model by Patel et al. Study has been conducted for Rayleigh number ( Ra) 10 4 to 10 7 while solid volume fraction ( ϕ ) of copper particles in water varied from 0.05% to 5%. It has been observed that the heat transfer decreases with increase in ϕ for a particular Ra, while it increases with Ra for a particular ϕ. The copper nanoparticle diameter has been taken as 100 nm for all of the studies.