Modern supercomputer performance is principally limited by power. TSUBAME-KFC is a state-ofthe-art prototype for next-generation TSUBAME3.0 supercomputer and towards future exascale. In collaboration with Green Revolution Cooling (GRC) and others, TSUBAME-KFC submerges compute nodes configured with extremely high processor/component density, into non-toxic, low viscosity coolant with high 260 Celsius flash point, and cooled using ambient / evaporative cooling tower. This minimizes cooling power while all semiconductor components kept at low temperature to lower leakage current. Numerous off-line in addition to on-line power and temperature sensors are facilitated throughout and constantly monitored to immediately observe the effect of voltage/frequency control. As a result,TSUBAME-KFC achieved world No. 1 on the Green500 in Nov. 2013 and Jun. 2014, by over 20% c.f. the nearest competitors.