Abstract: An off-line measuring method of void formation conditions was designed based on the characteristics of
the hot press process for resin matrix composites and the mechanism of void formation cont rolled by diffusion. This method can quantitatively determine the relation between resin pressure and porosity. By means of this method , the effect s of gel temperature , relative moisture and resin pressure on void formation conditions in two epoxy systems were studied , including void content , morphology and distribution. The modified Kardos void model was compared with the experiment s. The result s indicate that the method can be used to simulate the void formation process caused by hygroscopic moisture in the hot press process , and it has good repeatability. The evolution of resin pressure vs porosity follows the exponential decay function. Moreover , the gel temperature and relative moisture have great effect s on porosity , while the resin type influences porosity , morphology and dist ribution of voids remarkably. The study provides an important measuring method and experimental evidence for void defect cont rol in composites during the hot press process.