The CaCu3Ti4O12 samples with slight amount of doped bismuth were prepared and tested in this research. No second phase with bismuth was found in the doped samples. It was inferred that the bismuth ions has entered the lattice and take the place of the calcium ions. The grain size was diminished with the increase of the bismuth content. The bismuth atoms can inhibit the grains from growing large, and it could be used to modify the microstructure of CaCu3Ti4O12 ceramic. The reduction of the grain size resulted in the decrease of the relative dielectric constant, according to the “internal barrier layer capacitance (IBLC)” theory. The impedance measurements showed that the doped samples have less conductivity and lower potential barrier at the grain boundaries, and the substitution of the bismuth ions on the calcium cites might be the reason for it. As a result of the lowered potential barrier, the non-ohmic I-V property of the CaCu3Ti4O12 ceramic almost disappears in the doped ones.