Natural-convection cooling of a heat source placed at the bottom wall of a square cavity filled with non-Newtonian fluid was investigated numerically. Two thermal conditions were assumed at the source (imposed temperature or flux density). The effects of Rayleigh number, rheological index, n, source length, SL, its position, D, and Prandtl number, were analyzed. For a centered source. The three first parameters were varied. The results show an increase in dynamic and thermal fields’ perturbations when Rayleigh number increases and/or source length increases and/or n decreases. These observations are clearer in the first heating type compared to the second. Mean Nusselt number increase is recorded when Rayleigh number increases and/or n decreases, while it decreases with source length increase until SL = 1.0 where a new increase is recorded. For a non-centered source, the previous observations still valid, noting the loss of symmetry. The highest mean rate of heat exchange is recorded when D = 0.0 with decreasing amounts when moving towards the center. Finally, Prandtl number effect is analyzed. Results show a better diffusion of heat when increases for low n. Consequently, a marked increase in the mean Nusselt number for n < 1.0, weak for n = 1.0 and almost absent for n > 1.0 are recorded with a clearer effect for the first type of heating.