This paper describes the joining of aluminum and alumina by using polymethylphenylsiloxane, which is a type of polysiloxane. The polymethylphenylsiloxane that was coated on alumina formed an interlayer through a heat reaction involving aluminum, alumina, and polymethylphenylsiloxane. Scanning electronic microscopy and transmission electronic microscopy showed that the alumina and aluminum were joined together through the interlayer without any cracks or exfoliation, and the thickness of the interlayer was approximately 100 nm. The interlayer formed at temperatures of 873K and higher. The X-ray diffraction pattern and energy dispersive X-ray spectroscopy suggested that the interlayer consisted of aluminum silicate. The average bending strength of the joined samples was 232 MPa. Although the strength of the samples decreased during a thermal cycling test under conditions of alternating exposure to heating at 423K and cooling at 233 K, the strength maintained a value of at least 160 MPa, despite the number of thermal cycles exceeding 100 times.