Laminar incompressible mixed-convective heat transfer in two-dimensional lid-driven cavity, filled with nanofluid CuO-water, is studied numerically. Eight different viscosity models are compared to investigate the enhancement in the heat transfer and the increase in the average Nusselt number. The point of view of each model essentially differs in terms of whether it takes various parameters such as temperature effects, Brownian motion of the nanoparticles, the radii of aggregated particles, and the volume-fraction of nanoparticles into account or not. The governing stream-vorticity equations are solved using a second order central finite difference scheme, coupled to the conservation of mass and energy. The main sensitive parameters of interest to investigate the viscosity models are chosen as volume fraction of the nanoparticles φ, and Richardson number Ri. The performance study of the viscosity models and the interpretation of the corresponding results of velocity components are done in a different range of φ and Ri.