Size effect of tunnel deformation has been studied in this paper based on the deformation mechanism of different structures of rock mass. Size effect on deformation of rock mass surrounding tunnels closely relates to deformation mechanism of rock mass. Deformation mechanism of surrounding rock

which is intricate and diverse because of structural planes in the rock

includes material deformation and structural deformation. Size effect of deformation also reflects in the material deformation and structural deformation. Material deformation of surrounding rock varies directly as cross-section dimension of tunnels. Size effect of structural deformation is much more intricate. On the one hand

cross-section dimension of tunnels affects structure type of surrounding rock which closely relates to deformation mechanism; while amount of deformation provoked by different deformation mechanisms varies a great deal. On the other hand

in a same surrounding rock

amount of deformation provoked by an identical deformation mechanism also varies with size of tunnels. In the massive surrounding rock

for example

immobile rock surrounding tunnels will convert into movable rock in company with increase of cross section dimension of tunnels

and sliding or rolling deformation occurs sequentially. In the layered surrounding rock

bending deformation of rock stratification varies directly as the square of cross-section dimension of tunnels.