The thermal, mechanical, dielectric, and dynamic mechanical properties of polymer nanocomposites strongly depend on nanoparticles' content and aspect ratio. In this study, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)/cellulose nanowhisker (CNW) composite films with various CNW concentrations (0-4.6 wt %) were prepared by solution casting. Bulge, tensile, and dynamic mechanical tests showed that mechanical properties of the films increased with increasing CNW content until the content reached 2.3 wt %. Real permittivity of the composites also peaked at 2.3 wt % CNWs over a wide spectrum of frequencies (0.01-10(6) Hz). These property transitions occurring at 2.3% CNW content were due to the transition of CNW dispersion from homogeneous dispersion to agglomeration. Rheological results of the composites, however, indicated a transition point lower than 2.3%. This was attributed to the existence of a polymer-CNW network in the composite melt, which required no physical touching or geometrical overlapping between CNWs.