An experimental and numerical study was performed to investigate the macroscopic spray structure and the spray characteristics of common rail type high-pressure injector for the diesel engine. The global spray structure and microscopic characteristics of high-pressure diesel injector were investigated when fuel was injected at various injection pressures. Spray developing process and spray tip penetration were obtained by particle motion analysis system, and the quantitative spray characteristics such as Sauter mean diameter (SMD) and axial mean velocity were measured by using the phase Doppler particle analyzer. To consider the difference between primary and secondary breakup, KH-RT hybrid model, in which it is assumed that Kelvin-Helmholtz and Rayleigh-Taylor instability cause primary and secondary breakup, respectively, is used. After optimizing the breakup constants based on the axial distribution of SMD, the comparisons between predicted and experimental results were conducted. (C) 2002 Elsevier Science Ltd. All rights reserved.