Wood fibers from seven Eucalyptus species were collected to investigate the relationships among species, fiber biometry, and nanomechanical properties. The results indicated significant differences in wood density, coarseness, fiber length, fiber width, and cell wall thickness among the different Eucalyptus species. The nanomechanical properties of the S-2 cell wall layer also showed significant differences among the Eucalyptus species. The elasticity modulus ranged from 16 to 19 GPa, the hardness spanned 0.24 to 0.31 GPa, and the ductility ratio was between 54 and 68. Moreover, significant correlations were observed for hardness versus cell wall thickness (r = 0.87), and elasticity modulus versus crystallinity index (r = 0.80) and crystallite size (r = 0.68). Among the evaluated species, E. dunnii showed the highest elasticity modulus, highest hardness average, and the highest crystallinity index. The range of nanomechanical values indicated that Eucalyptus wood fibers are suitable for the development of new composite materials or engineering products by selecting the most adequate species for each use according to its properties.