In this work, the intrinsic drawbacks of polyhydroxybutyrate (PHB) such as slow crystallization rate, secondary crystallization and brittle nature were improved by blending with bio-based fillers, i.e. nanofibrillated/microfibrillated cellulose (NFC/MFC). A novel chlorinated-solvent-free based system was developed to blend PHB and NFC/MFC that resulted in homogenous dispersion of fibers in the PHB matrix, without the need for surface modification of fibers. The developed nano/micro-composite materials were fabricated as masterbatch pellets and films. Additionally, the effect of different NFC/MFC fiber morphologies influencing the crystallization behaviour of PHB was investigated in detail by differential scanning calorimetry, polarized optical microscopy and Fourier transform infrared spectroscopy. Both non-isothennal and isothermal crystallization studies (modelled with Avrami's kinetics) were performed on nanocomposites and variations in crystallization kinetics of PHB after addition of NFC/MFC were determined. Addition of NFC/MFC resulted in the drastic increase in the crystallization rate of PHB and hence they acted as nucleating agents. The fine and homogeneous morphology of NFC produced smaller PHB spherulites and restricted the growth of secondary crystals, hence resulted in more flexible films than PHB or PHB-MFC films, as determined by the mechanical testing of films. The more heterogeneous morphology of MFC altered the PHB crystallization mechanism most, as seen from the distorted shape of PHB spherulites along with the higher Avrami exponent, i.e. n >= 3.