Low dielectric constant glass-ceramic composite for high speed LSI packaging substrates has been developed by introducing isolated pores. Pore structure control can be achieved through the addition of polymer spheres to ceramic powders followed by subsequent thermal decomposition. The dielectric constant of the glass-ceramic composite, composed of quartz glass, cordierite and borosilicate glass having thermal expansion coefficient matching with that for Si chips, can be reduced to 2.9 by the introduction of 26% porosity. When applying such porous materials to multilayer substrates, it is important to consider how pores are isolated and whether or not they are sphere shaped, small and uniformly distributed, in order to prevent degradation in dielectric and mechanical reliabilities. However, the isolated porosity limit is about 13% and the dielectric constant can be reduced to 3.4. Besides, application to glass-ceramic composites composed of quartz glass and borosilicate glass has resulted in reduction in the dielectric constant (3.2). In addition, by using of a green sheet lamination technique, low dielectric constant multilayer glass-ceramic substrate with wiring was developed. The propagation delay can be reduced to 6.5ns/m.