Hybrid rockets use different phases of oxidizer and fuel as the propellant to obtain thrust. Hybrid rockets have advantages such as higher safety, lower cost compared to conventional type rockets, and restart ability. However, they have a disadvantage of low thrust. In FY2019 research and development of our rated thrust 50kgf class engine, the target thrust was achieved and the engine development was successful. On the other hand, the launch of the rocket powered by the engine failed due to insufficient initial thrust in the launch experiment. In this study, authors aim’s is to develop a small hybrid rocket engine with a rated thrust of 70kgf class, using LOX as an oxidizer and PMMA as a fuel and to increase the thrust by improving the design of the internal parts of the engine without changing the size of engine casing. Static firing tests and performance evaluations of the engine were conducted. As a result, (1) a target thrust of 686N (70kgf) was achieved by increasing the fuel port inner diameter, nozzle throat diameter, and the oxidizer mass flow rate. (2) Compared to the 50kgf engine, there was a decrease in combustion pressure and thrust with time. The reason of the decrease in the combustion pressure and thrust with time is that the relatively fast decrease of He gas pressure for pressurization which was caused by the faster decrease LOX residual due to the increased LOX flow rate.