This study is an investigation of preeminent fuel regression rate in Axial-injection End-burning hybrid rockets under high pressure conditions. The authors overcame a back firing problem at high pressure conditions by redesigning and optimizing the fuel port diameter. The fuel grain outer diameter was 38 mm and the port diameter was 0.4 mm. Firing tests were conducted using gaseous oxygen as oxidizer at chamber pressures range from 0.98 MPa to 1.44 MPa. The results of four static firing tests show that fuel regression rate increases as the chamber pressure increases, which is consistent with previous research results. Fuel regression rate reached approximately 12.6 mm/s at 1.44 MPa. The authors reformulated the relationship between fuel regression rate and chamber pressure based on the results of high chamber pressure region, and showed that the pressure exponent increased from 1.09 to 1.20.