It is generally said that mountain tunnels are little damaged by earthquakes. However, recent case studies of the damage of maintain tunnels caused by earthquakes also show that they are likely to be damaged when 1) the scale of earthquake is large, 2) there are earthquake faults near the tunnel or 3) there are special conditions. We collected infatuation on the tunnels which suffered damage from earthquakes, and performed field surveys of the tunnels damaged by the 1995 Hyogoken Nanbu Earthquake to study the damage mechanism of mountain tunnels. We analyzed the collected data, classified damage patterns and preformed simulation analyses and model tests. Based on the study results, we concluded that we can classify damage patterns of mountain tunnels by earthquakes into the following three patterns; 1) damage of tunnel entrance, 2) damage of tunnel at the fracture zone, 3) damage by sliding of faults. We can prove these mechanisms by the results of simulation analyses and model tests.