Abstract: The effect of ex-situ method on the mechanical properties and toughness of the RTMable carbon fiber (G827)／polyimide(PI-9731)composites was studied. The results show that the interlaminar shear strength of G827／PI-9731 composites toughened by the ex-situ methods with powder and film increases from 97.9MPa to 108MPa and 110MPa, respectively, when the thermoplastic PI mass fraction is 15% at room temperature. However, the interlaminar shear strength changes slightly at high temperature (288℃).The mode Ⅰ fracture toughness of G827／PI-9731 composites toughened with powder method is improved from 310J/m2 to 410J/m2. The Mode Ⅱ fracture toughness is improved from 590J/m2 to 939J/m2. However the mode Ⅰ fracture toughness of G827／PI-9731 composites toughened with film method increases to 459J/m2. The mode Ⅱ fracture toughness is improved to 1100J/m2. By SEM analysis, it is found that the toughness improvement results from the introduction of thermoplastic PI and the formation of the TP/TS phase inversion structure in the interlaminar regions, in which the PI-9731 particles wrapped by thermoplastic PI will be orientated and deformed in the process of crack extension.