In this study, two types of polymeric nanocomposite blends based on poly(l-lactide) (PLLA) and a thermoplastic polyurethane were prepared by reactive and non-reactive melt mixing. Multiwalled carbon nanotubes (MWCNTs) participated in reactive melt mixing and changed in both cases the morphology of blends. Melt rheology was used to study the influence of reactive melt mixing and localization of MWCNTs in terms of melt viscoelastic properties and relaxation time spectrum. Two relaxation times were observed for both types of blends due to reptation and stress release mechanism, although blends prepared by reactive melt mixing showed another relaxation mechanism associated with the generated interphase. Solid-state viscoelasticity determined by dynamical mechanical analysis showed significant influences of both MWCNTs and reactive melt mixing on storage modulus of the blends. Thermogravimetric analysis showed several degradation steps for polymer blends that contrast with the single step determined for both pure PLLA and PLLA nanocomposites.