SiC-based boundary layer capacitors were prepared by hot pressing. XRD, TEM and the high-resolution TEM techniques were used to characterize the sintered samples. It was found that the width of the grain boundary within the SiC-based boundary capacitors was about 200 nm. Extremely high dielectric constant of >2,400,000 appeared in a wide temperature range from 590oC to 730oC, with the maximum of >2,900,000. The critical temperature was about 500oC. Space charge polarization was detected as the temperature increased. Nano grains in the boundary phase were observed, which might enhance the space charge behavior.