The numerical experiment of the transient natural convection in two-dimensional cavity with non-Newtonian fluid is presented. In non-Newtonian viscosity models it is assumed that the laminar viscosity can be expressed as a continuous function of the strain rate. The present work is based on the power law model, which is the most popular non-Newtonian viscosity models. The results of numerical experiments of natural convection in two-dimensional cavity with Newtonian or non-Newtonian are presented.