This paper presents an assessment on aging model of IEEE and IEC standards using thermal model of oil-immersed power transformer for natural ester and mineral oil. For this purpose, a model created for the behaviour of transient thermal performance with aging of natural ester and mineral oils. Thermal model of the transformer is based on thermal-electrical analogy that is calculated separately for natural and mineral oils covering top-oil and hot-spot temperatures. The hot-spot temperature values of each ester/oil are used to calculate aging parameters which include the aging acceleration factor, time dependent relative aging rate and insulation life loss variations. In this study, mineral oil and natural ester are kept under temperatures which are 100℃ and 140℃ in different periods. Natural ester’s characteristics are higher than mineral oil’s, except its breakdown voltage. In addition, natural ester absorbs more water than mineral oil so it is understood that deformation of the insulating paper in natural ester is lower. Natural ester has important benefits to decelerate aging of insulation. There is a model which is proposed for natural ester oil–immersed transformers and there should be used an aging factor as R to predict remaining life.