The effect of firing temperature on microstructure and dielectric properties of ceramic substrate was studied. The substrate which consists mostly of Al₂O₃ was fabricated using tape casting technique. Three ceramic substrate samples were fired at different temperatures of 1100, 1300 and 1500°C. All samples were characterised for dielectric properties. Pore characterisation was carried out using small angle neutron scattering technique, and X-ray diffraction was used to determine grain growth. The present investigation shows that pore/grain boundary relaxation is the foremost reason of dielectric loss in this Al₂O₃ substrate in the low frequency region and grain size effect (interfacial grain boundary) was concluded to be less significant. This pore/boundary relaxation is found mostly originated from intragranular pore.