Polyimide (PI) short fiber-filled ethylenepropylenediene monomer (EPDM) insulations for a solid rocket motor were fabricated by surface modification of PI short fibers in an alkaline aqueous solution and subsequently by mixing formulation of EPDM insulations on a two-roll mill. The effects of PI short-fiber surface modification and short-fiber content on the mechanical and ablative properties were investigated. The excellent mechanical and ablative properties of PI fiber-filled EPDM insulations were based on their unique fiber/polymer adhesive property because of the rough surface character of modified PI fibers. The microstructures of char layers of the insulations were also characterized by scanning electron microscopy and energy-dispersive spectrometry.