Biodegradable poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)]/silica nanocomposites were prepared by melt compounding. The effects of silica on the morphology, crystallization, thermal stability, mechanical properties, and biodegradability of P(3HB-co-4HB) were investigated. The nanoparticles showed a fine and homogeneous dispersion in the P(3HB-co-4HB) matrix for silica contents below 5 wt%, whereas some aggregates were detected with further increasing silica content. The addition of silica enhanced the crystallization of P(3HB-co-4HB) in the nanocomposites due to the heterogeneous nucleation effect of silica. However, the crystal structure of P(3HB-co-4HB) was not modified in the presence of silica. The thermal stability of P(3HB-co-4HB) was enhanced by the incorporation of silica. Silica was an effective reinforcing agent for P(3HB-co-4HB), and the modulus and tensile strength of the nanocomposites increased, whereas the elongation at break decreased with increasing silica loading. The exciting aspect of this work was that the rate of enzymatic degradation of P(3HB-co-4HB) was enhanced significantly after nanocomposites preparation. POLYM. ENG. SCI., 2012. (C) 2011 Society of Plastics Engineers