K2O 6TiO2whiskers have been used to prepare 6061 alloy and pure A1 matrix composites by a squeeze casting process. The flexural strength and microhardness test have been performed for as-cast composites and in the T6 treated state. The interfacial microstructures of composites and the effect of interface reaction on composite properties have been studied with high resolution transmission electron microscope (HRTEM), energy dispersive spectrum (EDS) and X-ray diffraction (XRD). The results showed that the flexural strength of 6061A1 composites was 518 MPa in the as-cast state and decreased to 490 MPa in the T6 state. The tensile strength of pure A1 matrix composites was 238 MPa (as-cast) and 212 MPa ('T6'). In K2O. 6TiO2/A1 composites, the whisker was found to react with matrix easily, thus forming a continuous interlayer. In the case of 6061A1 matrix, the interlayer consisted of TiO, MgTi2O4, MgAl2O4, some 10-20 nm in thickness, which increased a few nanometers after T6 treatment. In the case of pure A1, besides a TiO layer, α-KO2particles were found near the interlayer. The degradation of composite strength following the T6 treatment was a result of the interfacial reaction which consumed the whiskers more intensively.