<p>The current research work is an attempt made to investigate the effect of defect in fatigue behaviour of ultrasonic machining tool using finite element method. In general, tools are discarded in the production quality check if any external defects are observed. However, the internal defect detection is a challenging task. Thus, under certain condition internal defect may not be detected in the tool production process. In this scenario, the tool with the internal defect may be allowed to use in the ultrasonic machining process. The machining process involves high oscillation with frequency of the tool and the horn ranging from 20 to 30&#xa0;kHz with mechanical contact of the tool with the abrasive materials. In such condition, the tool having an internal defect may fail before the predicted tool life. Thus, this study tries to understand the fatigue behaviour of the ultrasonic machining tool with different geometries in presence of internal defect in order to estimate the fatigue life using numerical simulation. The results are compared with the tool without having any internal defect and the comparison shows promising outcome in terms of change in the fatigue behaviour of the tool with defect than the tool without any internal defect.</p>