Abstract：Friction mechanism and frictional strength of soils are analyzed in this paper. Through a series of experiments，the external friction between soils or between soil and concrete and the internal friction of soils are studied. The results show that (1) the primary factors influencing external friction of soils include the grain com- position of soils，the water content，the chemical properties of the pore water and the vertical load on soils，(2) the apparent and intrinsic internal friction factors reduce as the water content in soils increases，and the influence of the water content on the former is larger than that on the latter，(3) the suctions between grains (especially the variable structure suctions) in soils can exert a tremendous influence on the friction of soils，and (4) the influence of water content on external friction strength between structure surfaces is different from the influence on internal friction strength of soils. The distinguish between internal friction (factors) and external friction (factors) of soil is mainly that the formers belong to apparent ones，comprising the contribution of coherences such as generalized suctions that are relative to chemical character of aqueous solution and saturation state，and the latters belong to intrinsic ones，comprising the contribution of coherences such as generalized suctions of shear strength. The coherence and internal friction，which constitute the compressive strength of soil mass，are correlative.
