This study deals with the interfacial reaction of aluminum borate whisker reinforced aluminum alloy composite prepared by squeeze casting process. By means of transmission electron microscopy（TEM） observation，it has been found interfacial reaction occurred under the as cast　condition．This reaction produced the spinel（MgAl2O4）．As　composites were reheated under various conditions，it has been found interfacial reaction occurred and grown of the spinel was proportional to reheated time and temperature．However growth rate of the spinel in the composites reheated in argon or in air was the same．By means of scanning electron microscopy（SEM）observation，it has been found that aluminum alloy reacted with the whisker， therefore the whisker deteriorated and Mg contents in the aluminum alloy matrix decreased． The effects of interfacial reaction on the mechanical properties of the composite were studied with the bending test and micro-vickers hardness test．As a result，the whisker deteriorated decreased the bending strength，but did not decrease the micro-vickers hardness of the composite．