The aim of this study is to develop and characterize carbon fiber (CF)-reinforced polyetherimide (PEI) composite materials that can be used in aerospace and defense industries. Four different composite materials with 60/40 and 40/60 resin/fiber weight ratios were prepared by hand lay-up using unidirectional (UD) CFs and TWILL CFs. It was determined that, the thermal stability of composite materials is independent of CFs. PEI resin and composite materials soften above 200 degrees C and decompose above 500 degrees C. Also, PEI resin did not lose mass in 10 years under isothermal conditions at 25 degrees C, and it is estimated that it loses 3 x 10(-8)% of its mass after 10 years at 100 degrees C. Mechanical analysis showed that, tensile strength and Young's modulus of UD CF-reinforced composite materials were similar to 3 times higher than that of the TWILL CF-reinforced ones. The mechanical service lifetime studies under 10 MPa load at 100 degrees C showed that, only 30-50% of mechanical properties of composite materials are lost in 10 years. Although, the thermal and mechanical aging studies were carried out under more challenging conditions, these composite structures showed strong mechanical and thermal behaviors. In conclusion, all composites, especially PEI-UD60, are promising to use in ammunition body, wings, and fins where excellent tensile strength, high stiffness, lightness, thermal stability, and renewability are desired requirements.