We show that the magnitude of the Dzyaloshinsky-Moriya interaction (DMI) can be determined by a field-driven domain-walls (DWs) annihilation measurement. It is predicted that the DMI induces antiparallel Neel DWs which form a metastable 360 degrees DW when they approach to each other. We find that an annihilation field, over which two DWs collide and vanish, is proportional to the magnitude of the DMI. This result suggests that the DMI can be quantified by a simple measurement of the annihilation field. (C) 2014 The Japan Society of Applied Physics