As a kind of reinforcing agent, the application of nanocrystalline cellulose (NCC) is widely limited in hydrophobic polymers owing to its rich hydroxyl surface. In this study, NCC was modified with lauric acid/p-toluensulfonyl chloride mixture, then the modified nanocrystalline cellulose (mNCC) was incorporated into biopolyester poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3,4)HB) by solution casting to prepare P(3,4)HB/mNCC nanocomposites. The prepared mNCC and P(3,4)HB/mNCC nanocomposites were characterized by Fourier transform-infrared, X-ray diffraction, contact angle test, transmission electron microscopy, scanning electron microscopy, differential scanning calorimetric, polarized optical microscope, dynamic mechanical analysis, and thermogravimetric analysis. The results show that the crystallinity and mechanical properties of P(3,4)HB are greatly improved due to the fact that NCC can be modified successfully and the mNCC can distribute uniformly in nanoscale in the matrix with good compatibility along the interface. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2015-2022, 2013