A stochastic trigger by micro-turbulence for neoclassical tearing modes (NTMs) is studied. NTMs induce topological change of magnetic structure and have a subcritical nature. It is found that the NTM can be excited regardless of the presence of seed islands. The transition rate of the NTM, its probability density function and its statistically averaged amplitude are obtained. The boundary in the phase diagram is determined as the statistical long time average of the transition conditions. The NTM can be excited by crossing this boundary even in the absence of other global instabilities. The boundary is expressed in terms of the poloidal beta value. Its dependence on the ratio between the ion banana width and radius of the rational surface, ρb/rs, is found to vary in the range ρb/rs–(ρb/rs)2, depending on the linear stability of the tearing mode.