A development study of an air precooling system called 'a precooler' for the air-turbo ramjet engine with expander cycle (ATREX engine) is summarized here. Three models of precooler has been manufactured and tested by the ground combustion tests step by step. The latest model could be operated without critical damages during the cumulative time of 1,610 sec in the test number of 23. However, the thick frost formed on the cooling tube surfaces, which caused an additional pressure loss and decrease of a heat exchange rate. Some countermeasures against the frost formation were devised, in which two active methods were tried. One is to inject the cryogenic fluid such as LN2 and LOX, which decreases to supply the vapor into the cooling surface due to the molecular diffusion. It can compensate 40% of the pressure loss due to the frost formation by using 0.2 kg/s of LN2 mass rates for 1 kg/s of the airflow. The other is to inject condensable additives such as methanol, which eliminates the frost and/or increases the frost density. It can compensate 80% of the pressure loss by 0.01 kg/s of methanol for 1 kg/s of the airflow. Defrosting effects is related to the diameter of injected methanol particles as well as uniform mixing. The smaller diameter is better because surplus methanol particles cannot easily attach to the cooling tubes by inertia force.