Abstract: The performance of two types of PMI foam is studied by tests, including physical, mechanical properties, moisture absorption, compressive creep and outgassing in thermal vacuum environment. A comparison between foam core and aluminum core is made. Advantages and disadvantages of PMI foam in space structures are analyzed. With its advantages, such as, good dielectric performance, low thermal conductivity, and machinability, PMI foam can be applied in the structures of spacecraft, including multi-functional structure, variable-section structure, rod & beam, and antenna, etc. After being properly reinforced, PMI foam can be used for the primary structure or the secondary structure of spacecraft.