Novel nanocomposites based on aliphatic poly(ester-ether) [poly(p-dioxanone), PPDO] and fibrous clay (sepiolite) were successfully prepared by surface-initiated polymerization of p-dioxanone (PDO) in the presence of organo-modified sepiolite (OSEP). The existence of OSEP-grafted PPDO in nanocomposites, generated via surface-initiated polymerization from the hydroxyl group-abundant OSEP surface, greatly contributes to improving the performance of these nanocomposites. The dispersion uniformity of OSEP nanofibers in PPDO matrix was confirmed by scanning electron microscope (SEM) observations. Crystallization rate increased with increasing OSEP loading. Surprisingly, only OSEP nanofibers bundles acted as heterogeneous nucleating agents during the latter stage of PPDO crystallization. The incorporation of OSEP significantly enhanced the modulus and viscosity of nanocomposites, whereas nanocomposites still presented liquid-like behavior as neat PPDO. The exciting aspect of this research is that nanocomposites were reinforced and toughened by the addition of OSEP nanofibers.