This paper describes the mechanical configuration of the Science Instrument Module(3.4) (SIM) used on the Advanced X-ray Astrophysical Observatory (AXAF-I). The SIM houses the focal plane scientific instruments of AXAF and provides instrument selection and focusing capabilities. It is designed to meet a large number of requirements of both the science instruments and the observatory. An overview of the SIM is provided and a brief description of the components and their functions is presented.