This article provides a methodology for calculating the health index for oil-paper insulation system in power transformers with rated voltage between 69kV and 230kV, using fuzzy logic.The article also provides a detailed review of the existing limits of acceptance, according to the present standards and for each of the six input variables: moisture content, acidity, dielectric strength, dissipation factor oil, dissolved gas and furan content.With this information, the membership functions are obtained and the six input values are related to the output value, which represents the health index of the transformer.Using data provided by a company of the Colombian electricity sector for 18 power transformers the health index is calculated and the results are analyzed.The health index could be useful for establishing a ranking of the state of power transformers with the aim of better assigning resources and maintenance activities.