An experiment is conducted for testing the greatest strength of herb root system meanwhile measuring the root diameter under the condition of the greatest tensile strength. It provids where root was tore out and the potential root tensile strength is caculated. The result was revealed that the root average tensile strength vetiver grass of 85 mPa common cetipede grass of 27.3 mPa white clover of 24.6 mPa Juncelles serotinus of 24.5 mPa paspalum dilatatum poir of 19.7 mPa Bahio grass of 19.2 mPa manila grass of 17.5 mPa Bermuda grass of 13.4 mPa. The shear strength test shows that the soil plus vetiver grass root made the shear strength increased, compared to the CK in sandy or clay soil. The feature of holding up soil of herb root network is discussed and the potential capacity of holding up soil ie. the tensile strength of root network is put forword as one of the most important factor. It justifies the basic mechanics theories substituding soft bioengineering approach for the hard engineering approach in completely or partially to hold up soil and slope and resistant against soil erosion and control the landslide and mud rock flow etc.