The bio-mechanics of 12 normal grasses root with diameter of 0.1~1.0 mm in Loess Plateau are studied by using of simple device designed by author. The result showed that the grasses root have very strong tensile strength. The grass root, with diameter of 0.1~0.2 mm had the anti-tensile strength of 1. 37N, legumes 0.77N and Artemisia of 0.69N. The tensile strength of hair root increased with its diameter enlarged and the incensement rate is that grass >legumes>Artemisia. The effect of age on root tensile strength vary with the plant species, however, there was no signified difference among the species of Artemisia. It was found that there exist a logarithm relations between stress and strain for Astragalus adsurgens and Medicago sativa in which the strain of hair root had a fast raising rate than the prediction by Hooker s Law, and the rest of hair root follow the law with different modulus of elasticity. The rank of elongation rate were Artemisia>Ispedeza duvurica>Setaria virideis. The sensitiveness of stress-strain to strain decreased with the root diameter increased. The critical strain rate varied with root diameter had three types: increase progressively; decrease successively and one-kurtosis curve. The significance of characteristic of bio-mechanics for plant hair roots in improving soil anti-scourability were discussed in the paper.