A study on strength decrease due to water contents through infiltration has been conducted with 9 rocks of high frequency in Korea. It has been proved that a strength decrease through infiltration has high dependence on rock strengths ranges while the species of rock have no impact. It has been found that the weaker the rocks are, the more sensitive the strength decreases are, and that water content of 0.5% corresponds to strength decrease of almost 50%. Another finding is that most rocks have a failure when water content is about one quarter~one half of its saturation. It has been shown that the weakening of rock strength due to water content results from leaching and the weakening of bonding strength of cementation materials.