The rheological properties of a series of microbially synthesized poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)s (PHB-HHxs), with varying comonomer (HHx) content, were systematically investigated. Shear viscosities show dependence on the rate of deformation, temperature, molecular weight, and copolymer compositions. The zero-shear viscosity h 0 follows the classical M 3:4 w power-law relationship with the weight average molecular weight M w . The characteristic relaxation time l, which indicates the onset of shear thinning, ranges from 0.02 to 0.2 s for different PHB-HHxs and is roughly linearly related to h 0 . The temperature dependence of rheological properties follows an Arrhenius form. Activation energies for flow E a are obtained from the slope of the natural logarithm of the shift factor a T plotted against the inverse of temperature curve, and the values for PHB-HHxs are found to be in the range of 27-36 kJ/mol E a decreases with HHx content in the copolymer, a trend that can be related to the difference in chemical structure between HHx and HB, according to the method of Vankrevelen and Hoftyzer. A Generalized Maxwell model models the viscoelastic behavior of the PHB-HHx melt well. The value of the plateau modulus G 0 N obtained suggests a highly entangled configuration. The molecular weight between entanglements M e decreases from 11,600 to 9400 as HHx content increases from 3.8 to 10.0 mol%. Our results suggest that the presence of propyl groups in HHx increases the steric hindrance of the PHB-HHx chains, thus resulting in increased segmental friction and entanglement density. As a result, viscoelastic parameters for PHB-HHx copolymers, such as h 0 and G 0 N , are readily tunable by varying the HHx content, making them attractive as ''green'' substitutes for non-degradable thermoplastics.