To improve the product quality of rotate vector (RV) reducer at a low cost, a kind of vibration technology is introduced to investigate the quality assurance of RV. It includes a torsional vibration test to assess the vibration performance of the reducer and vibration analysis to recognize the manufacturing defects in the reducer’s mechanical components. The mechanical components with manufacturing defects are diagnosed through the signal processing on torsional vibration and defect characteristics recognition, and they are removed from the RV reducer by a reassembling process. As a result, the dynamic behavior of the reducer is evidently improved, the torsional vibration of the reducer is kept within a low level, and the feedback control is realized for product quality during the production process. Therefore, the presented vibration technology is highly recommended to the manufacture quality assurance of RV reducers applied in an industrial robot.