The correlation between nanostructure, as derived by X-ray scattering, and the micromechanical properties, as revealed by microindentation hardness, of a series of novel random ester-amide copolymers, based on butyl terephthalate and an etherdiamide, has been investigated. The interface distribution function has been used for the analysis of the small-angle X-ray data. The influence of the etherdiamide content and the number of the flexible groups in the etherdiamide unit on the nanostructure of the copolyester-amides is discussed. Hardness is shown to be a property critically dependent on the number of etherdiamide sequences within the copolymer chains. The hardness values of the copolyesters appear to deviate from the linear additivity law. This deviation seems to be mainly due to a contribution of several factors: (i) the lower hardness value of the poly(butyl terephthalate) (PBT) amorphous segments within the copolymer with respect to that of the PBT homopolymer; (ii) an increase in the surface free energy of the PBT crystals with increasing etherdiamide content; (iii) a decrease in the energy required for the mechanical deformation of the PBT crystals with increasing etherdiamide content and increasing number of flexible groups within the etherdiamide unit.