Helium bubbling is known as a cooling and an anti-thermal stratification method of cryogenic liquid. Cryogenic liquid is evaporated into helium bubbles by a partial pressure driving force during the helium bubbling. Based on this phenomenon, the present thesis investigates helium bubbling as a pressurization system for a liquid rocket. Key factors to determine the characteristic of helium bubbling are investigated. With KSLV-II upper stage, it was reviewed about the possibility to apply helium bubbling as a pressurization system. It is discerned that the helium bubbling has the minimum weight as a pressurization system in the specific system condition. Helium bubbling tests for liquid oxygen were performed with a vessel of 4 mm diameter and 4 m length according to the operating conditions such as the tank pressure (1~ 3 bar(a)), the level of liquid (0.3~3 m) and the flow rate of helium (0.1~0.3 g/s). The results of the thermodynamic equilibrium model meet a good agreement with the experimental data on the condition that the level of liquid oxygen is not less than 1 m. But the results of the thermodynamic equilibrium model disagree with the experimental data on the condition that the level of liquid oxygen is less than 1 m. The non-equilibrium model with the correction factor (α) applied in the equilibrium model is proposed. The correction factors are suggested with the experimental data performed on the condition that the level of liquid is less than 1 m, and it is confirmed that the correction factor is only dependent of the level of liquid. The proposed empirical correlation for the correction factor is $-12.96+exp(-level(m)/0.07644)+0.99$. With the proposed model, the parametric studies were conducted about the helium bubbling as a pressurization system applied to the KSLV-II upper stage according to the variation of tank pressure, flow rate of helium and total loaded mass of liquid oxygen, and helium bubbling was compared to 500 K hot gas system in terms of the needed helium mass. The specific operating conditions for which helium bubbling is the most effective as a pressurization system are presented. The boundary of the operating pressure that distinguishes the effective zone of each system as a pressurization system is located in 2.17 ~ 2.45 bar (a). In case that the operating conditions of system is located under the suggested boundary, 90 K helium bubbling is more effective than 500 K hot gas system. As the length/diameter (L/D) of tank becomes larger, the effective zone of 90 K helium bubbling increases. But the variation of L/D (0.54~3) has little effect on the movement of the boundary. As the loaded mass of liquid oxygen increases, the effective zone of 90 K helium bubbling increases. It can be determined whether 90 K helium bubbling is effective or 500 K hot gas system is effective on the specified operating conditions with the boundary presented.