Abstract: The tensile strengths of saturated and dry concrete under different loading rates were studied by splitting tensile tests. The experimental results show that the tensile strength of saturated concrete was lower than that of the dry concrete under low loading rate but was larger under high loading rate. The comparison between dry and saturated concretes indicates that the free water in the pores and cracks affects the mechanical properties of concrete. The mechanisms of these changes are analyzed in terms of the acting modes of pore water on the saturated concrete under different loading rates. The free water in the cracks acts as a wedge, which decreases the strength of saturated concrete under quasi-static loading, but it acts as opposite force, which increases the strength of saturated concrete under high loading rate according to the Stefan effect in physics. The fracture mechanics based on these mechanisms are used to discuss these changes and the results show that these mechanisms can explain the mechanical properties of saturated concrete well.