Abstract: For the colossal dielectric constant mechanism of calcium copper titanate, the most widely accepted model is an inner barrier layer capacitance model. It is said that in polycrystalline calcium copper titanate ceramic, its semiconducting interior grains are surrounded by thin insulating grain boundaries, i.e. internal barrier layer, which has great influence on its dielectric properties. Results of scanning electron microscope (SEM) show that the sample grain boundary is jam like. It is hard for SEM to measure the thickness of the insulating internal barrier layer in grain boundary region. So positron annihilation technique (PAT) was used to characterize the thickness of the internal barrier layer in this work. The research was carried out by co-doping various concentrations of Al/Nb on its Ti⁴⁺ sites to change the microstructure of calcium copper titanate ceramic grain and grain boundary （CaCu₃Ti_4-xAl_0.5xNb_0.5xO₁₂, x=0.2%, 0.5%, and 5.0%）. PAT results show that the change trends of sample mean lifetime and Doppler parameter S are similar. Besides, the mean lifetime, Doppler parameter S and the longer lifetime component are minimum at x=0.5%. This implies the thinnest thickness of its barrier layer. The obtained experimental results support the prediction of internal barrier layer capacitance model that describes the origin of the colossal dielectric constant for the calcium copper titanate ceramics.