In this work, 2D and 3D models are compared for the thermal simulation of a power transformer. The experimental work from a heat run test of a national manufacturer is used to validate the models. The continuity, Navier-Stokes, and energy equations were used to simulate oil flow within the windings and core of the power transformer. A commercial CFD software was used to solve the numerical equations resulting from the discretization of the domains of the 2D and 3D models. The 3D model provides a better description of the oil flow inside the windings and the core of the power transformer, and it shows a zone of almost stagnant oil flow in the vertical cooling channels and also it shows an area of high temperature of oil in the winding located farthest from the oil inlet.