The interface microstructure between α-silicon nitride whiskers and the 6061 aluminum matrix in a squeeze cast composite was observed by transmission electron microscopy. No reaction product was found at the interface. Only a strain field in the matrix near the interface was observed. The strain field induced a thin layer along the interface in which the aluminum lattice was distorted. This layer occasionally looked like an amorphous layer when observed under the exact Bragg diffraction condition of the matrix, but it seemed not to be really amorphous. After prolonged reaction at 800°C, the whiskers reacted with the matrix to produce aluminum nitride and silicon.