Abstract: The thread of drilling tools is subjected with large and complicated stresses in drilling process, and it easily tends to generate fatigue cracks due to stress concentration. When the stress concentration develops to a certain extent, it will make a failure of the thread of drilling tools, and then this may bring serious impacts on the production. Repairing the thread is one of the effective ways to reduce the stress concentration. But the workshop is always blind to the repairing of the thread, and it lacks technical guidance to test. According to the theory of MMMT(Metal Magnetic Memory Testing), one can use MMMT to analyse the differences of the stress concentration of the thread before and after repairing, and to verify the effectiveness of the thread repairing. Also one may judge what state of the stress concentration of the thread is. In short, one may offer the guidance for the workshop to repair the thread.