The fundamental studies of PEI/carbon fiber thermoplastic composites have been carried out to understand the correlation between the moisture penetration phenomena of laminates and processing variables for consolidation process such as the mold temperature and pressure. It has been found that the relationship between mass uptake arid square root of diffusion time in the initial stage of diffusion is linear This moans that the diffusion phenomena of PEI/carbon fiber thermoplastic composites follow the Fickian diffusion. The initial diffusion rate and equilibrium mass uptake values were increased with reducing mo]d temperature and pressure due to the debonding and delamination of laminates. A similar tendency has been obtained with increasing water temperature and applied stress. From the optical micrograph, the delamination and debonding phenomena by the water diffusion have been confirmed. The effects of processing parameter on structural defects have been found to be the decrease of mechanical properties and tile increase of void content.