Composites materials made of biobased matrices reinforced with natural fibers are a solution considered to meet the environmental constraints of industry. Interactions between a partially biobased epoxy resin and flax fibers were investigated using Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis type trials (DMA). DSC analysis revealed that the catalyst effect of water increases the variation of entropy and lead to higher Eα values. DMA measurements highlight that composite with wet fibers has a higher Tg value after manufacturing and it is a consequence of the higher reactivity induced by water. By contrast, composite reinforced by dried flax fiber showed a lower curing reactivity and by consequence lower Eα values and Tg value after manufacturing. Both composites were stored under relative humidity conditions after manufacturing and evidenced an increase of Tg due to water absorption. A post-curing helped the composite with dried fiber to reach maximum Tg while it wasn’t the case for composite with wet fibers due to chaotic network geometry caused by the entropic effect. Water contained by the flax fibers can act as a plasticizer after the post-curing as the Tg decrease with the absorbed water. This work gives a better understanding to the role of water naturally contained by flax fibers and point out the fact that monitoring the moisture of flax fibers is a key parameter to manufacture biobased composites.