Abstract. Injection performance of negative-ion based NBI system for JT-60U has been improved by correcting beamlet deflection and improving spatial uniformity of negative ion production. Beamlet deflection at peripheral region of grid segment has been found due to distorted electric field at the bottom of the extractor. This was corrected by modifying the surface geometry at the extractor to form flat electric field. Moreover, beamlet deflection due to beamlet-beamlet repulsion by space charge was also compensated by extruding the edge of the bottom extractor. This resulted in reduction of the heat loading on the NBI port limiter. Uniformity of negative ion production was improved by tuning filament emission current so as to put more arc power in the region where less negative ion current were extracted. As the result of improvements above, the continuous injection of 2.6 MW H0 beam at 355 keV has achieved for 10 s. Thus the long pulse injection up to nominal pulse duration of JT-60U was demonstrated. This has provided a prospect of long pulse operation of negative-ion based NBI system for steady state tokamak reactor. So far, the maximum injection power of 5.8 MW at 400 keV with deuterium beam and 6.2 MW at 381 keV with hydrogen beam have been achieved in the JT-60U N-NBI.