The effect of water on fracture toughness in various modes of mudstones was investigated using semicircular bend (SCB) specimens exposed to three-point bendings. Natural mudstone specimens are obtained using a special coring method and are then classified into three types (A, B, and C) corresponding to three directions/configurations of bedding planes (divider, arrester, and transverse). The results show that the Type A (divider configuration) specimens possess the largest fracture toughness value for all tested modes and same soaking time, whereas the Type C (transverse configuration) specimens have the smallest one. By increasing soaking time, the fracture toughness in all three modes decreases and the fracture mechanism changes from brittle failure to ductile failure. Among them, the Type C specimens have the highest degree of degradation for each soaking time period. Regarding the fracture modes, the degradation degree ofK_Icis higher than that ofK_IIcfor all three types of mudstone specimens. In addition,K_IIc/K_Icratio increases when soaking time is extended. Furthermore, in the initial and short soaking time stages, the experimentalK_IIc/K_Icresults are consistent with theoretical findings from modified MTS criterion. However, after being soaked 300 minutes for three types of specimens, the test curves deviate from the theoretical curves. Analogously, the mixed-mode I/II ratio ofK_efftoK_Icis consistent with the theoretical values in the initial stage when the degree of damage is low. With soaking time increasing, the experimental curve is gradually deviated from the theoretical curve. When soaking time reaches 300 minutes, the deviation is substantial. And the test data for the Type-A specimens are observed to provide better agreements with theoretical predictions by modified MTS criterion than those for the other two types of specimens.