Abstract: The SEM was employed to determine the phase structure of the toughened epoxy system, both in-situ and ex-situ system, respectively. The results of the in-situ blending system indicate that the phase reversion occurs when the thermoplastic (TP) content is 16.7%~20.0%. Spherical particle dimension of the phase-separated system was statistically analyzed, showing that the higher the TP component, the smaller the dimension and the narrower the dispersion of the spherical particle sizes. An experimental expression was obtained to establish the relationship between the TP content (φ) and the particle dimension (d). Based on this expression it is estimated that the upper and lower limits of the particle dimension are 1.838μm and 0.925μm, respectively. For the ex-situ toughened resin, a multi-layer structure, i.e. toughened layer -interface layer -bulk layer, is formed. The content of TP keeps a high level in the toughened layer, followed by a dramatically drop in the adjacent plies; hardly any TP resin exists in the bulk layer. Also, the interface is expected of a relatively narrow thickness to achieve preferable toughness.