Abstract: In this paper the fracture energy G_IC of epoxy-PSF system has been tes- ted with TDCB (Tapered-double-cantilever beam) samples.The results show that this system not only has a high G_IC value, but also G_IC and G_Ia are almost equal.Comparing the G_IC temperature char with dynamic mechanical properties(DMA),we find that the toughness of the system relates to its visco- elastic properties.From the SEM photographs,a structure that the tenacious polymer as an unintrruptad phase, wraps thermosetting spherule, i.e.a"networ- kmicrospherule" structure was observed.It is this structure that makes the G_IC of epoxy-PSF system high.