It is estimated that the gear oil is exposed to very severe thermal conditions when Shinkansen electric cars run in the 350km/h speed range. Under such severe conditions, it would be difficult to keep the performance of current mineral type gear oil stable for a long period. Therefore, we have developed two synthetic gear oils with PAO (poly-a-olefin) as a base oil. We have investigated the performance of the synthetic gear oils through various laboratory and field tests. As a result, it was proved that the synthetic gear oils have excellent performance such as thermal oxidation stability and a high load carrying capacity.