The work-roundign mechanism by centerless grinding under a stable working condition is studied so far mostly from a geometrical point of view. Recently, Dr. Gurney and Rowe applied the regenerative theory to the case of centerless grinding and pointed out the existence of unstable grinding regions. In these papers, however, the work-rounding mechanism as well as other phenomena in an unstable region are not made clear yet. In the present paper, are made clear, taking the regenerative effect due to work-surface waviness into consideration, the generation mechanism of self-excited vibration that the given grinding condition which is stable geometrically transits to an unstable region as a result of work-surface regeneration and system dynamic characteristics, and the mechanism that the vibration is kept at a steay state. That is, analyzing the stability criterion as for a grinding system by the help of vector loci, the work-rounding mechanism under the self-excited vibration is clarified. Then, it is confirmed experimentally that there is a case where the self-excited vibration is not generated even though the given grinding condition is situated in an unstable region according to the solution by Gurney and others.