Abstract: Both tandem motor and lengthened motor are novel mud motors developed to improve the performance of conventional screw drill,and their working mechanisms are basically the same,which aims to obtain high power and high torque output by increasing motor series.Based on the brief introduction of conventional mud motor,the working mechanism,basic structure,performances and developing tendency of the tandem and lengthened motors were illustrated,together with their benefits also presented.As the conventional tool for extended reach wells,they have improved ROP remarkably due to their higher torque,and this also cut down the drilling cost dramatically.