Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) is a thermoplastic biopolyester with a slow crystallization rate. L-Phenylalanine (L-PH) was used to improve the crystallization rate of PHBHHx. The nonisothermal crystallization kinetics and melting behaviors of PHBHHx and PHBHHx/L-PH samples were characterized and compared with differential scanning calorimetry. At cooling rates of 2, 5, 10, and 20 degrees C/min, PHBHHx/L-PH could crystallize at higher temperatures than pure PHBHHx. A modified Avrami equation based analysis showed that the addition Of L-PH shortened the half-time of crystallization of PHBHHx from 9.6 to 8.0 min and increased the composite rate constant of PHBHHx from 0.201 to 0.283 during the nonisothermal crystallization process at a cooling rate of 10 degrees C/min. For other cooling rates, similar trends of changes were observed. These indicated a faster crystallization rate of PHBHHx in the presence Of L-PH. PHBHHx/L-PH samples showed a higher melting temperature and a sharper melting peak than those of the pure PHBHHx, which suggested that L-PH as a nucleation agent improved the perfection of the PHBHHx crystals. Stress-strain measurements showed that both PHBHHx and PHBHHx/L-PH maintained their ductile and elastic properties during the 60 days of the aging study. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 2950-2956, 2008