Dense and uniform Sip/LG5 composites were fabricated by squeeze casting process, and then were treated by high temperature diffusion. Microstructure observation indicated that Si transformed from irregular sharp-particles to 3D network structure after high temperature diffusion treatment. Si-Al interface of 3D-Sip/LG5 composites is smoother and has better bonding degree, compared with the interface of the untreated composites. There are Si precipitation in the interface and the matrix of 3D-Sip/LG5 composites. Twin crystals were observed in the matrix of 3D-Sip/LG5 composites. The average linear thermal expansion (CTE) of the composites is decreased by 10.5% after high temperature diffusion treatment. However, the thermal conductivity of 3D-Sip/LG5 composites does not change significantly due to the decreased Si precipitation in the interface and matrix by 3D network structure of Si.