Root tensile strength is an important factor to be considered when choosing suitable species for reinforcing soil on unstable slopes. Experiments were conducted for testing root tensile strength and measuring root cellulose content of Medicago sativa and Digitaria sanguinalis. Results indicated that in the two species, both the tensile strength and percentage of cellulose were negatively correlated with root diameter, while a positive correlation was found between percentage of cellulose and tensile strength. The conclusion confirmed that the relationship among root tensile strength, root diameter and root cellulose content found in tree root system was held also in the two species. The research demonstrated also a negative relationship between tensile strength and root lignin/cellulose ratio, and the influence of cellulose content was greater than that of lignin content on root tensile strength. There exists a logarithm relationship between stress and strain for Medicago sativa root, which does not follow Hooker's Law, and Digitaria sanguinalis root follows the Law. The experiment also testified that both the sensitiveness of stress-strain and critical strain increased with the decreasing root diameter. With improved root tensile force and diameter test apparatus, the experimental results were more reliable and accurate. The present research gained an insight into the soil reinforcement mechanism of the herb root system and the selection and of plant species for the purpose of soil and water conservation.