High-pressure injection is effective in reducing diesel smoke. However, much is not known about selecting the best combination of the spray number and the nozzle-orifice diameter at high injection pressure. To make this clear, effects of the orifice diameter on the spray characteristics at elevated injection pressures are assessed in detail based on current spray theories. The results suggest that at a smaller orifice diameter, a rise in the injection pressure increases much faster than otherwise, not only the flow rate of air in entrained in terms of the fuel flow rate but also the microscopic mixing rate within the spray, thereby preparing a leaner and less heterogeneous mixture and ensuring a less smoky combustion. Theoretical background is given for the finding that a nozzle having more orifices of smaller diameter should be combined with a lower swirl intensity for high-pressure injection.