A fundamental study on spray behavior for small quantity injection under high boosting and high temperature conditions was carried out by applying dual nano-spark shadowgraph photography method to constant volume spray chamber filled with inert gas. The study showed macro-scale and micro-scale spray structures and dynamic changes of diesel spray affected by ambient density and temperature right after the pilot injection. Detailed observation and analysis of the images showed that high density surrounding gas decreases the mainstream velocity of the spray after injection, which restrains air entrainment into the spray. There is a change of flying direction of the droplets at spray boundary under high density condition. Many droplets move outward from the spray after the lapse of time. Spray evaporation is promoted by vaporization of droplets at spray boundary. High density condition greatly promotes droplets evaporation at spray boundary. However, at high density condition, large size droplets tend to remain at a long time at the vicinity of injector outlet after small amount of injection.