Currently in development at Muroran Institute of Technology, a small gas-generator-cycle air turbo ramjet engine (GG-ATR) for a supersonic unmanned aerial vehicle requiring deep throttling uses liquid oxygen/ethanol as a propellant. However, it is difficult to maintain a liquid-phase flow at a low mass flow rate or at a low pressure due to flashing in the feedline. In response, we investigated the ability of our ground test facility to feed liquid oxygen at a low mass flow rate. The experimental results of the flow tests validated the proposed one-dimensional model, including heat input and the density change in the feed line. The minimum possible flow rate as a subcooled liquid flow was examined with the model and the throttling level of 75 % was possible when assuming 110 K in tank temperature.