In this article, poly(ether ether ketone) (PEEK) with pendant fluorocarbon groups was synthesized to impart better impact resistance to diglycidyl ether of bisphenol-A (DGEBA) epoxy resins cured with 4,4 0 -diamino-diphenyl-sulfone (DDS). Mechanical properties were investigated as a function of the molecular weight and contents of PEEK. It was found that both impact strength and tensile strength varied with molecular weight and contents of PEEK. In the system of epoxies modified with M n 24,000 PEEK, drastic decline rather than increase of impact strength and tensile strength was observed, while impact strength was increased by 17% for the system of epoxies modified with M n 9500 PEEK at a relatively low PEEK content. To reveal the toughening mechanism, phase morphology of the modified epoxies were examined with scanning electron microscopy (SEM) and a two-phase morphology was observed. Ductile fracture mode was observed for the modified epoxies and a rigid particle toughening mechanism was proposed. An image analysis software Quantlab-MG was used to analyze the dimension parameters of the dispersed particles for the epoxies modified with M n 9500 PEEK. A larger and more dispersed distribution of particles at higher thermoplastic content was found. Dynamic mechanical analysis (DMA) was employed to further study the phase behavior of M n 9500 PEEK modified epoxies. The observed phenomenon was interpreted by the reaction-induced phase separation mechanism. On the basis of the rubber elasticity theory, an indirect measurement of crosslinking density was taken and the results indicated that the incorporation of PEEK might reduce the crosslinking density of epoxies. V