<p>In this study, the flow characteristics of hydrogen ejector were numerically analyzed by varying the diameter and volumetric flow rate of the nozzle and mixing section of the ejector to improve the entrainment performance and minimize the energy loss. The volumetric flow rate of the ejector was varied from 20 to 110 LPM, and the diameters of the nozzle and mixing section (<i>D</i>_n, <i>D</i>_m) were varied by 11% and 25%, respectively. The Mach number (Ma), pressure (<i>P</i>), and turbulence kinetic energy (TKE) were analyzed, and internal energy loss (<i>E</i>_loss) was evaluated. The results showed that shock waves occurred at 60 LPM and above, and the entrainment ratio decreased. Compared to the base geometry condition at 60 LPM, the TKE was reduced by up to 17.83% and the exergy destruction ratio by up to 16.57% for <i>D</i>_n = (+)11% and <i>D</i>_m = (+)25%, respectively. This result confirms that it is a factor that can reduce irreversible losses inside the ejector. The correlation between internal energy losses and entrainment performance was then evaluated. It was found that the <i>D</i>_n = Base, <i>D</i>_m = (+)25% condition minimized the internal energy loss and showed high entrainment performance.</p>