The existence of sclerophyllous plants has been considered an adaptive strategy against different environmental stresses. As it literally means “hard-leaved”, it is essential to quantify the leaf mechanical properties to understand sclerophylly. However, the relative importance of each leaf trait on mechanical properties is not yet well established.Genus Quercus is an excellent system to shed light on this since it minimizes phylogenetic variation while having a wide variation in sclerophylly. Thus, leaf anatomical traits and cell wall composition were measured, analyzing their relationship with LMA and leaf mechanical properties in a set of 25 oak species.The upper epidermis outer wall had a strong and direct contribution to the leaf mechanical strength. Moreover, cellulose plays a critical role in increasing leaf strength and toughness. The PCA plot based on leaf trait values clearly separated Quercus species into two groups corresponding to evergreen and deciduous species.Sclerophyllous Quercus species are tougher and stronger due to their thicker epidermis outer wall and/or higher cellulose concentration. Furthermore, section Ilex species share common traits regardless of they occupy quite different climates. In addition, evergreen species living in Mediterranean-type climates share common leaf traits irrespective of their different phylogenetic origin.