This paper investigates the effect of confining stress on rock fragmentation under TBM cutters by numerical simulation. Firstly, based on the characteristics of the force-penetration curve, the confining stress conditions for TBM tunneling are divided into three general classes through an index of confining ratio defined as the ratio of confining stress to uniaxial compressive strength. Then, the rock fragmentation processes with different confining stress induced by TBM cutters are studied by using a micromechanics-based numerical code. It is found that the confining stress has significant impact on the key factors for rock fragmentation, including the chipping force, crack angle, effective crack length and energy dissipation. Specifically, the chipping force and the crack angle increase with the rising of confining ratio. However, for the effective crack length, there exists a critical confining ratio at which it reaches a minimum value. Hence, when confining ratio is larger than the critical value, it will promote rock fragmentation, and vice versa. For energy consumption, similarly, the same critical confining ratio is also found, at which the specific energy reaches its peak value. This study provided an explanation for the field observations, where special condition of confining stress will better facilitate fragmentation process by TBM cutters. (C) 2011 Elsevier Ltd. All rights reserved.