The spin tunnel junctions (STJ) using AlOx insulating barrier layer has been investigated and its composition and electrical characteristic are discussed. The AlOx insulating barrier layer was fabricated by ion-beam-sputtering Al with oxygen ions accelerated from assist gun. The barrier height was determined by measuring the current-voltage characteristic of Co (10 nm)/AlOx/Co (50 nm) STJ and fitting the current-voltage curve by Simmons' fitting. It is revealed that varying the oxygen gas flow at the assist gun changed the composition of AlOx and insulating layer characteristics, which can be explained by Anderson's model. The barrier height also depended on the sputtering time. Furthermore, the noise of STJ fabricated by this method was lower than the noise of STJ using insulating barrier layer fabricated by natural oxidization. From these experimental results, It is expected that by optimizing the oxygen gas flow at the assist gun and the sputtering time, we can obtain Co/AlOx/Co STJ with high magnetoresistance ratio and low noise.