A recent trend in high-pressure gasoline pumps is increasing the outlet pressure. One of the most important topics for increasing this pressure is improving volumetric efficiency. Therefore, the purpose of this research is to quantify the breakdown of efficiency loss factors and to suggest a new design for improving volumetric efficiency. Authors developed a method of quantifying the efficiency loss breakdown of high-pressure gasoline pumps by using 1D fluid pressure simulation results and conducting evaluation experiments regarding sensitivity. Authors separated pump movement into three phases; suction, compression, and delivery. Authors then investigated the loss factors in each phase. As a result, authors obtained an equation for predicting the final output volume. The equation consists of a limit output volume and other types of leakage volumes. The limit output volume is calculated from the volume sucked into the pump chamber, output pressure, dead volume of the pump chamber, and displacement volume. Finally, authors analyzed the efficiency loss breakdown of a previous pump design by using the developed method. According to the analysis, the major loss factor was reduction in effective displacement volume due to the dead volume of the pump chamber. This effect becomes stronger under high pressure. Based on this result, authors suggest a new low-dead-volume pump chamber design. Authors predict that the new design can maintain volumetric efficiency over 60% up to 30MPa delivery pressure.