In this paper, numerical simulations are made to study the influence of different excavation sizes and different joint set dip angles on the deformations of tunnels excavated in fractured rock mass by using the code of UDEC. It is assumed that there are two types of joints, i.e., joints with horizontalvertical dip angles (0°-90°) and joints with inclined dip angles （45°-135°）, in the influenced domain. Conclusions drawn are as follows: 1) The deformation of the surrounding rocks of tunnels in rock mass with horizontalvertical joint sets grows slowly as the excavation size of the tunnel increases before the deformation grows dramatically and failure occurs. The failure takes the form of integral collapse of surrounding rocks above the crown toward the ground surface; 2) The deformation of the surrounding rocks of tunnel in rock mass with inclined joint sets grows slowly as the excavation size of the tunnel increases before the deformation grows dramatically and failure occurs. The failure takes the form of local collapse of surrounding rocks at the crown; 3) The deformation of the tunnels excavated in rock mass with inclined joint sets is larger than that of the tunnels excavated in rock mass with horizontalvertical joint sets. 