The effects of ultra-high injection pressure (P(inj) = 300 MPa) and micro-hole nozzle (d = 0.08 mm) on flame structure and soot formation of impinging diesel spray were studied with a high speed video camera in a constant volume combustion vessel. Two-color pyrometry was used to measure the line-of-sight soot temperature and concentration with two wavelengths of 650 and 800 nm. A flat wall vertical to the injector axis is located 30 mm away from the injector nozzle tip to generate impinging spray flame. Three injection pressures of 100, 200 and 300 MPa and two injector nozzles with diameters of 0.16 and 0.08 mm were used. With the conventional injector nozzle (0.16 mm), ultra-high injection pressure generates appreciably lower soot formation. With the micro-hole nozzle (0.08 mm), impinging spray flame shows much smaller size and lower soot formation at the injection pressure of 100 MPa. The soot formation is too weak to be detected with the micro-hole nozzle at injection pressures of 200 and 300 MPa. With eliminating the impact of injection rate on soot level, both ultra-high injection pressure and micro-hole nozzle have an obvious effect on soot reduction. Soot formation characteristics of impinging spray flame were compared with those of free spray flame using both the conventional and micro-hole nozzles. With the conventional nozzle, flat wall impingement deteriorates soot formation significantly. While soot formation characteristics of free spray flame with the micro-hole nozzle are not altered obviously by flat wall. Liquid length of the 0.16 mm nozzle is longer than the impingement distance and liquid length of the 0.08 mm nozzle is shorter than the impingement distance. Liquid impingement upon the wall is responsible for the deteriorated soot level of impinging flame compared to that of free flame with the conventional nozzle. (C) 2010 Elsevier Ltd. All rights reserved.