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Investigations on the piezoresistive effect of poly-silicon nanofibns (PSNFs) have not been presented, since it is considered that their piezoresistive properties can become worse with film thickness decreasing. However, our experimental results indicated that the PSNFs (~100nm in thickness, even thinner) had a high gauge factor (>30) and low temperature coefficients of resistance and gauge factor, suited for sensing applications. In order to study the effect of preparation parameters on the response linearity and time drift of the PSNF resistance, PSNFs with different doping concentration were deposited by LPCVD to fabricate sensing resistances. The film microstructure was characterized by SEM, TEM and XRD. The film resistivity and resistance change rates under different loading strains were measured. Based on the modified trap model, a conclusion was drawn that the response linearity and time drift of resistances were strongly depended on the carrier occupation level of trap states at grain boundaries. The conclusions are significant to improve the accuracy and reliability of piezoresistive sensors.