The formation and evolution of the initial spray structure of diesel fuel in the near-nozzle region under different injection and ambient pressures were studied. A visualisation experiment study on diesel fuel using long-distance microscopy and nanosecond-pulse flashlight was performed. Four types of spray tip structure were identified and named as ‘needle’, ‘bubble’, ‘mushroom’ and ‘umbrella’. The obtained high-resolution images revealed that both injection pressure and ambient pressure had a significant influence on the evolution of the spray tip structure. The measurement of the spray penetration and spray angle showed that the increase of injection pressure enhanced spray dispersion while the increase of ambient pressure exhibited an opposite effect. In order to provide a better understanding on the formation mechanism, a numerical study based on large eddy simulation (LES) and volume of fluid (VOF) interface capturing technique was conducted.