Natural fiber-reinforced composites (NFRCs) suffer from water absorption and low temperature stability, resulting in fiber degradation and subsequent material failure. Built-in piezoresistive sensors are investigated to monitor the deformation/strain of the component. As a low-cost material from renewable resources biochar particles derived from olive stones were applied on flax plies and yarn bundles that served as model systems. Carbon black samples as petrochemical variants were used as a reference material. Biochar and carbon black-covered fiber systems were laminated in epoxy resin followed by tensile tests. The electrical resistance was recorded simultaneously during testing. Biochar with a broad size distribution from nano to high micrometer range (D < 200 mu m) was superior in sensor performance compared to carbon black and biochar with a smaller particle size range D < 20 mu m. Gauge factors (GF) of NFRC samples with integrated biochar particles reached 30-80 while carbon black could not exceed a GF of 8. To obtain maximum GFs, yarn count of flax yarn/ply substrate should be as thin as possible, but still enable percolation of the adhering particle network. Comparatively large particle size was identified as a contributing factor enabling the high GF for coarse biochar compared to carbon black.