Abstract: The flowing and infiltration behaviors of the equivalent fluid (resin), which passed through the thickness direction (z direction) of large thickness un-toughened and "ex-situ" toughened preforms were studied. The pressure variation of inlet and outlet pot were monitored respectively by pressure transducers during z direction flowing RTM (z-RTM) process. Based on the pressure curves, the macro-flowing and micro-infiltrition behaviors of the "resin" inside the "ex-situ"toughened preforms and untoughened preforms were deduced. The results show that the resin flow along the z direction between the gap of fiber bundles and at the same time infiltrated the intra fiber bundles during the injection stage of z-RTM process. The "ex-situ" toughener will delay the resin macro-flowing, however the resin flow front will be smoothed by the interlaminar toughener. Compared with the un-toughened preform, the introducing of interlaminar toughening agent reduces the z-direction permeability of the preform from 2.9×10^-14m² to 3.5×10^-15m².