Generally there are three kinds of substances used as negative-ion generator in textiles, natural silicate minerals (ceramic/tourmaline), natural rare-earth minerals and natural sediment with ultra-fine pores. Based on different additive, the mechanism of negative-ion generation is largely divided into three kinds, the piezoelectricity and pyroelectricity of tourmaline crystal, air ionization caused by low-level radiation and breaking up of the clusters of moisture in air when going through the ultra-fine pores of natural sediment. In this paper, the negative-ion generating properties of natural fiber fabrics-cotton, wool, silk and linen were first proposed. By some kind of physical stimulation, rubbing or vibrating, natural fiber fabrics without any additive could also emit negative ions. Considering that the piezoelectric effect was observed in wool fibers, silk fibers and cellulose fibers, the piezoelectricity was studied as a mechanism of negative-ion generation of natural fiber fabrics. Another possible mechanism was the triboelectricity produced by the sense of rubbing or vibrating and tip discharge of hairiness. The final experiment results verified that the latter would be the main reason, and the electrolytic dissociation of moisture was also ontributing to negative-ion generation.