A high temperature superconducting power cable requires forced flow cooling. Liquid nitrogen is circulated by a pump and cooled down by a subcooling system. Typical operating temperature range is expected to be between 65 K and 80 K. The circulating liquid nitrogen is subcooled by liquid nitrogen boiling on the shell side of a subcooler heat exchanger, and then it cools the HTS cable. The HTS power cable needs sufficient cooling to overcome its low temperature heat loading. This loading typically comes in two forms. One is heat leak from the surroundings and the other is internal heat generation, i.e., AC loss. This paper describes performance tests of the cooling system. Heat exchanging performance of the subcooler, pressure drop between supply and return lines, and heat transfer coefficient of a former are examined for the performance test.