The influence of polymorphism on the micromechanical properties of isotactic polybutene-1 (iPBu-1) has been investigated by means of the microhardness technique. Hardness data, H, of form I (hexagonal) are shown to be notably larger than those of form II (tetragonal). The H values of both polymorphic forms are shown to depend on the molecular weight, M-w, and the crystallization temperature, T-c. The hardness behaviour with M-w and T-c has been correlated to the changes of degree of crystallinity and nanostructure as derived from small angle X-ray scattering. The hardness values for iPBu-1 infinitely thick crystals (Hc(c)(infinity)), of forms I and II, have been calculated for the first time. H-c(infinity) of form I is shown to be notably larger than that of form II. This result is a consequence of the denser packing of the hexagonal crystal modification, and accounts for the large difference in H values found for forms I and II. Finally, the variation of the mechanical b-parameter, proportional to the surface-free energy of the crystals, with molecular weight is discussed.