The piezoresistive behavior of a silicon carbonitride ceramic derived from a polymer precursor is investigated under a uniaxial compressive loading condition. The electric conductivity has been measured as a function of the applied stress along both longitudinal and transverse directions. The gauge factor of the materials was then calculated from the data at different stress levels. The results show that the material exhibits an extremely high piezoresistive coefficient along both directions, ranging from 1000 to 4000, which are much higher than any existing ceramic material. The results also reveal that the gauge factor decreases significantly with increasing applied stress. A theoretical model based on the tunneling-percolation mechanism has been developed to explain the stress dependence of the gauge factor. The unique piezoresistive behavior is attributed to the unique self-assembled nanodomain structure of the material.