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We investigated the relationship between the microstructures and dielectric properties of various CaCu3Ti4O12 (CCTO) polycrystalline ceramics sintered in air. An abrupt increase in the dielectric constant (εr) from ~3,000 to ~170,000 at 1 kHz occurred with increasing the sintering temperature from 980 to 1000°C for 12 h, respectively, which was accompanied by a very large increase in the average grain size from 5 to 300 µm, respectively, due to an abnormal grain growth. With further increasing the sintering temperature, the εr value at 1 kHz was slightly decreased to ~150,000 at 1020°C with no variation in the average grain size, significantly decreased to ~77,000 at 1040°C with a large decrease in the average grain size (~150 µm), and then maintained the values of ~76,000 and ~69,000 at 1060 and 1080°C, respectively, without noticeable variation in the average grain size. While no abnormal grain growth occurred in the CCTO samples sintered at 980ºC for the holding time to 24 h and thus their εr values showed relatively lower εr values (< ~4,000 at 1 kHz), the abnormal grain growth occurred in the samples after a certain holding time at a given sintering temperature of higher than 1000°C and thus their εr values abruptly increased. Analyses by the complex impedance (Z*) and modulus (M*) spectroscopy revealed that the εr values of the CCTO samples were dominantly affected by the electrical properties of grain boundary so that high εr values over 10,000 at 1 kHz were attributable to the high capacitance (C) of grain boundary, which is in good agreement with grain boundary internal barrier layer capacitor (IBLC) model.