Piezoresistivity of polymer-derived silicon carbonitrides made from a polysilazane modified with different amounts of thermal initiator is measured at room temperature. It is found that the thermal initiator has a significant effect on the electric conductivity, which first increases and then decreases with increasing thermal initiator concentration. The highly conductive sample exhibits much higher piezoresistive coefficient as compared with the low conductive samples. The microstructures of the materials are characterized using Raman spectroscopy. Based on these results, the piezoresistive behavior is described using the tunneling-percolation mechanisms, the piezoresistive effect of SiCN ceramics is controlled by the concentration and distribution of free carbon clusters, and the effect of thermal initiator on the formation of free carbon clusters in the materials is discussed.