Mechanical properties of binderless fiberboard are correlated to the hydrothermal history of the fibers during the board process. Water and temperature thickness gradients allow to calculate a gradient of T-Tg difference (7g, local glass transition temperature, function of local water content; T, local temperature). This gradient explains the variation of mechanical properties within the thickness and in time. The in situ plasticization of lignin, which is supposed to increase the T-Tg difference, improves the macroscopic mechanical properties of fiberboards.