[en] Dry twin screw vacuum pump, which generates no oil and gas during operation, inherits the advantages of screw machinery, such as high reliability, stable medium conveying, small vibration, simple and compact structure, convenient operation, etc. which has been widely used in petrochemical and new or high-technology industries, like vacuum heat treatment, nuclear research, micro electro mechanical systems, nanotechnology, precision manufacturing and national defense technology. In this paper, the geometric feature of the twin screw vacuum pump such as contact line, meshing line and chamber volume between teeth is analysed, respectively. In order to study the micro working process within the dry twin screw vacuum pump, a test rig has been designed and built and the p-V indicator diagrams of the working process of dry twin screw vacuum pump have been recorded successfully under various working conditions. Based on the indicator diagrams, the working process within the dry twin screw vacuum pump is analysed in more detail, and especially, the effects of the suction pressure as well as rotational speed on the performance of dry twin screw vacuum pump, such as pumping speed and ultimate pressure, is thus investigated. The results presented in this paper are helpful to deepen understanding of the working process inside dry twin screw vacuum pump and offers a good reference for to improve design and optimize such machines. (paper)