A liquid oxygen supply system for a small-scale sounding rockets was developed. The sounding rokets are hybrid type employing a combination of plastics (PMMA or high density polyethylene) and liquid oxygen as propellants. Key points for the miniaturization were using no valve in the liquid oxygen feeding line and omitting precooling treatment of the line. Without precooling treatment, the liquid oxygen in the feeding line becomes multiphase flow until the temperature of the feeding line falls below the boiling temperature of the liquid oxygen. A characteristic time of multiphase flow duration was proposed to evaluate the duration a multiphase flow holds in the feeding line. Static firing tests showed that the multiphase flow ends within the half the characteristic time, showing that the liquid oxygen flow rate history without a precooling treatment is acceptable for an actual operation of the rocket motor. Finally, an impinging type injector was developed to remove a combustion instability, caused by a coupling of the combustion chamber pressure and the propellant (liquid oxygen) feed system.