Ceramic samples of CaCu3Ti4O12 (CCTO) were prepared and the voltage–current characteristics were investigated. Extremely strong nonlinear behavior, similar to the reported results in literature [Nature Materials 3 774–8 (2004); Appl. Phys. Lett., 89 191907 (2006); Appl. Phys. Lett., 89 212102 (2006); Appl. Phys. Lett., 91 091912 (2007)] was observed. However, we found that this is a measurement artifact due to a thermionic emission current feedback amplification effect. Compared to that of ZnO varistor, the thermionic emission current of CCTO sample is about six orders of magnitude higher and it is feedback amplified due to strong negative temperature coefficient resistance behavior when the CCTO sample is tested with a voltage (or current) source. Eliminating the thermionic emission current feedback amplification effect, the true value of nonlinear coefficient of the CCTO sample is much lower.