Abstract: According to the requirments of liquid composite moulding, the compaction behaviors of two types of "ex-situ" toughened preforms were investigated. The preforms were fabricated by interleaving such "ex-situ" toughening layers as the porous film or the non-woven fabric between the carbon fabrics. The thickness of the preforms under different pressure was measured by the mechanical testing machine. The results show that the "ex-situ" toughening layers decreses the fiber volume fraction of the preforms under a certain pressure, and the compaction behaviors of the preforms,under different pressures, depend on the micro-structure of the toughening layers. Additionally, the thickness of the preforms was predicted by adopting a compaction constitutive model for the fabric. It is shown that the calculated results are in good agreement with the experimental results when the empirical coefficient k=2.