In this study, the effect of altitude on free diesel spray morphology, macroscopic spray characteristics and air-fuel mixing process was investigated. The spray visualization experiment using high-speed photography was performed in a constant volume chamber which reproduced the injection diesel-like thermodynamic conditions of a heavy-duty diesel engine operating at altitudes of 0m, 1000m, 2000m, 3000m and 4500m. The results showed that the spray morphology became narrower and longer at higher altitude, and small vortex-like structures were observed on the downstream spray periphery. Spray penetration increased and its angle decreased with increasing altitude. At altitudes of 0m, 1000m, 2000m, 3000m and 4500m, the spray penetration at 1.45ms after start of injection (ASOI) was 79.54mm, 80.51mm, 81.49mm, 83.29mm and 88.92mm respectively, and the spray angle was 10.9°, 10.8°, 10.7°, 10.4°and 9.8° respectively. The air-fuel mixing process was analysed using empirical relations like fuel spray area, spray volume, mass of air entrained within the spray and equivalence ratio. At altitudes of 0m, 1000m, 2000m, 3000m and 4500m, calculation from those empirical relations showed that at 60mm downstream of injector tip, the spray area was 329.73mm2, 327.16mm2, 322.37mm2, 314.93mm2 and 298.56mm2 respectively, and the spray volume was 1872.01mm3, 1843.19mm3, 1789.97mm3, 1708.84mm3 and 1536.87mm3 respectively, and the mass of air entrained was 34.0mg, 32.28mg, 29.52mg, 25.88mg and 19.49mg respectively, and axial equivalence ratio was 4.21, 4.33, 4.56, 4.92 and 5.82 respectively, indicating that the fuel-air mixing quality was deteriorated as the altitude increases. And also the average rates of change in the mass of air entrained and axial equivalence ratio with addition of altitude of 1000m were obviously larger than that in the spray area and spray volume when the altitude was higher than 3000 m. Finally, the air entrainment analysis provided some useful insights on the combustion of a diesel engine operating at different altitudes.