In materials that possess a well developed sub-grain structure in the deformed state, nucleation can be considered as a type of abnormal (sub-)grain growth. The critical condition for this type of nucleation was investigated with a Monte Carlo code. The findings were as follows. (1) The nucleus size is a key factor for survival of an embryo. An embryo with a size below a critical value is not able to grow into the matrix. If the initial radius becomes larger than the critical value, the radius has a small effect on the growth rate. (2) High stored energy is definitely necessary for both nucleation and growth. (3) In order to grow into the critical size, the neighboring subgrains must have a small disorientation with nucleus. If the disorientation of the surrounding subgrains, becomes large after growth of an embryo to critical size, the nucleus can grow into the matrix abnormally.