The whisker/matrix interfaces in a β-Si3N4w/60 61Al composite were structurally characterized by high resolution transmission electron microscopy (HRTEM). It was shown that there was a nearly amorphous layer (2–3 nm thickness) at a whisker/matrix interface. The magnesium segregation at the whisker/Al interface was revealed by electron dispersive analysis of x-ray (EDAX). MgO and MgAl2O4 nanocrystal particles were formed at the whisker/matrix interface due to the Mg segregation there during the manufacturing of the composite. The Mg2Si particles preferred to precipitate at the whisker/Al interface when the composite was processed with T6 heating treatment. There were specific orientation relationship of the MgAl2O4 or Mg2Si particles with the β-Si3N4 whiskers.