Experimental study was conducted to investigate the effect of ultra-high injection pressure on combustion and emission characteristics in a single-cylinder diesel engine. Electronically controlled ultra-high pressure fuel injection system consistently supplied the fuel of ultra-high pressure up to 250 MPa. Various injection pressures, 40 to 250 MPa, were applied and compared. A injector with eight identical nozzle holes which have diameter of 105 μm was used. The results showed high potential to improve the nitrogen oxide (NOx) and particulate matter (PM) trade-off relationship with an ultra-high injection pressure and the exhaust gas recirculation (EGR).