Sol-gel gamma -Al2O3 coating was deposited onto surfaces of aluminum borate whiskers to control the spinel reactions between whiskers and matrix alloys. High-resolution electron microscopy (HREM) observations of the whisker defects and coating structure in the squeeze-cast Al(18)B(4)O(33)w/gamma -Al2O3/6061Al composite were carried out. Stacking faults and edge dislocations in the whiskers were directly observed, and zigzag anti-phase boundaries were also found. A deformation of whiskers along sectional direction was observed, which was determined to be a source of in situ fracture of whiskers. The sol-gel gamma -Al2O3 coating was a nanocrystalline material with an average grain size of 5 nm.