In countries like South Korea, various studies to construct tunnels and underground structures are performed due to the existence of many mountains and rock-consisting underground space. Therefore lots of attempts have been tried to characterize the rock and the rock mass. However, it is very difficult to characterize rock mass for engineering purposes because the connections of discontinuities and fractures, which play a significant role in characterizing the rock mass are very complicated. As a result it is very hard to understand and predict various parameters of the rock mass. Among the various parameters of a rock mass, one of the most important parameter is the hydraulic conductivity of the rock mass in many areas of the civil engineering like geotechnical engineering and underground engineering. As mentioned, like many parameters of the rock mass the hydraulic conductivity is also hard to predict. Many studies were performed numerically, experimentally and theoretically to evaluate the hydraulic conductivity of the rock mass.