In this work a thermal-hydraulic analysis of the low voltage winding of a power transformer working in oil natural mode will be performed. The study considers the thermal and hydraulic balance of the cooling loop to determine the mass flow and oil temperature at the inlet of the winding. Balance of the windings and radiator determine the boundary conditions of the model. The thermal model of the winding is performed using Computational Fluid Dynamics. The winding simulations are made with a 2D axisymmetric model run with the commercial code ANSYS Fluent. Several phenomena appear in the winding, due its configuration, that affects the heat transfer in local regions, reducing the local hot-spots in regions where the oil mass flow is low. The results will be compared to those obtained when the mineral oil is replaced by a natural ester. For natural ester, the same phenomena appears with lower intensity due to the lower mass flow and the higher viscosity. The results of this study predicts 5.7ºC lower bottom-oil temperature and 48% lower mass flow rate at the winding for the natural ester compared to mineral oil. The reduction of mass flow through the winding is caused by the high viscosity of the natural ester. Also, the reduction of the mass flow leads to lower bottom-oil temperature. Regarding the winding model, mineral oil shows 7.3ºC lower hot-spot temperature than the natural ester case.