In this study, the experiments was carried out to investigate the effect of LN2 precooling temperature of 288K, 243K, 213K, 173K at 1 ~ 4bar of air liquefaction system. Experimental appratus on the air liquefaction system by using the LN2 precooling with direct cooling was designed and equiped. It was consisted of an air compressor, a GM refrigerator, liquefaction vessels, a liquid nitrogen precooler, and a vacuum pump. In case of the cooling without load, the temperature of GM refrigerator was decreased to 24K at 1st stage and 7K at 2nd stage, which meaned that the liquefaction vessel was well insulated and the cooling system was high performance. The experimental results showed that while increasing air pressure and decreasing the precooling temperature, the cool-down time was decreased significantly, and the cumulative liquefaction rate was increased about 22%. Also FOM was increased as increasing air pressure and decreasing the precooling temperature.