An optimal cooling method for a long HTS power transmission cable was scrutinized by using theoretical models. Cooling length of HTS cable is determined by pressure range and temperature range of LN2 in the HTS cable. Results showed limitation from temperature range is stricter than that from pressure range. The well-known one-side cooling was modified to two-side cooling. It was shown that cooling length can be nearly doubled by adopting two-side cooling of the same capacity.