The effect of microwave treatment on the electric conductivity and structure of a polymer-derived SiCN ceramic is studied. It is found that the conductivity of the microwave-treated sample is about 40 times higher than that of the conventional heat-treated one at the same temperature and dwell time conventionally. The X-ray diffraction patterns show that both samples are amorphous without obvious crystallization. Raman analysis reveals that the microwave-treated sample exhibited a narrower full width at half maximum and upper-shift of G peak. X-ray photoelectron spectroscopy spectra show that there is a significant sp(3)-to-sp(2) transition of free carbon in the microwave-treated sample. These results suggest that the microwave-treatment can induce a distinct structure evolution of the free carbon, which contributes to the remarkable enhancement of the conductivity of the sample.