The initial spray morphology reflects the initial nozzle condition (quantity and distribution of residual fuel) derived from the previous injection, which will considerably affect the spray primary breakup. High-speed microscopic imaging technique and a single-hole diesel injector with a transparent Plexiglas nozzle were employed to investigate the mechanisms for the formation of initial spray tip morphologies and the corresponding breakup characteristics under various conditions. It was found that the interaction between liquid fuel and air bubbles, fuel properties and pressure shock-wave strength determined the spray tip morphology. Depending on the initial conditions, several types of morphologies were observed, namely, compact mushroom spray induced by surface tension under initial air-free condition, thin mushroom spray due to the acceleration effect of compressed air under low common rail pressure, drum spray tip (single or double drums) caused by the catastrophic breakup of air bubbles under common rail pressure, and mushroom-drum and central jet under fuel-free condition. Low common rail pressure allowed more time for the air bubbles to be compressed and more energy reserved in the compressed air, resulting in a higher possibility of mushroom spray formation through the accelerating effect. However, strong shock-wave under high pressure caused a high tendency for the catastrophic breakup of the air bubble and thereby production of the dispersed drum. In addition, less residual fuel left by the previous injection under high common rail pressure significantly suppressed the formation of mushroom tip.