AZ91D magnesium alloy reinforced with aluminum borate whisker (Al₁₈B₄O_33w, denoted by ABO_w) was fabricated by squeeze-casting. The heat treatment (T4) was carried out at 693 K in argon atmosphere for 48 and 96 hours, respectively, following water-quenched. The behaviors of interfacial reaction were studied in processes of fabrication and following heat treatment. Microstructures of interfacial reaction layers were investigated with transmission electron microscopy (TEM). Interfacial reaction layer in thickness of 5–10 nm was found in as-casted composite, and it was identified that the interfacial reaction layer was consisted of MgO. When the composite was heat-treated at 693 K for 48 hours, the surfaces of whiskers were covered with interfacial reaction layer completely and the thickness of interfacial reaction layer increased to about 20 nm. With increasing the time of heat treatment from 48 hours to 96 hours, the thickness of interfacial reaction layer on the surfaces of whiskers did not change obviously. The interfacial reaction layer was consisted of tiny MgO particles and a small quantity of MgB₂ particles. Although MgO particles played a good barrier to keep out of contacting between magnesium and whisker, the boundaries between MgO particles acted as a shortcut of magnesium atoms migration. Therefore magnesium could keep on reacting to whisker. The interfacial reaction could invade into the inner of whisker about 20 nm depth after the composite was heat-treated at 673 K for 96 hours. The surfaces of whiskers became wave-like because of interfacial reaction. An orientation relationship was found between MgO reaction layer and ABO_w: (11\\bar1)_MgO||(320)_ABOw and [011]_MgO||[001]_ABOw. The surfaces of MgO particles were consisted of close-packed (111)_MgO plane.