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- This paper deals with the temperature characteristics according to the cooling medium and the duct size in model transformers. For the analysis and the temperature-rise tests, two 400kVA model transformers have been manufactured. One has been filled with the alpha oil as the cooling medium and constructed the duct sizes of 3㎜ and 5㎜ in the low-voltage and high-voltage windings respectively. The other has been filled the beta oil and the duct sizes were 4㎜ and 6㎜. The temperature-rise tests have been performed by the back-to-back method and the load factor has been controlled the range of 90%~130%. The temperature values have been measured by the thermocouple and from the sixteen points in each transformer. A commercial CFD program "FLUENT" has been used for the analysis of temperature distribution. The geometry of transformer has been modeled to 3-dimensional by using the hybrid calculation mesh including the radiator. And also, the natural convection velocity has been measured at the oil top position, and compared with the calculated results.