The development of 275-kV internally conductorcooled cable systems is under way to apply such systems to the long distance large capacity (1500MVA/cct) underground transmission.As the final step of the development, we have made a long term field test in Yokosuka Laboratory of the Central Research Institute of Electric Power Industry from February 1985 through May 1987.We could confirm that the field test proved the long reliability of the internally conductor cooled cables, the accessories and the cooling facilities, and they were available for practical use.From the point of the reliability and the energy saving of the systems, the development of the operation control system of the cooling facilities is especially important, because the facilities require large cooling capacity due to the large heat l o s s of the cable conductor. Therefore we have proved. in this field test, that the computer controlled operation system is very useful.In addition, we have developed the simulation program of the operation control system, and we have confirmed that the calculated values are in good agreement with the measured ones.