This paper investigates the effects of natural flours on the crystallization behavior of poly(3‐hydroxybutyrate‐co‐3‐hydroxyhexanoate) (PHBH). Two types of PHBH (3‐hydroxyhexanoate [3HH] contents of 5.6 and 11.1 mol %) were used as polymer matrix. One of two natural flours (cellulose or wood) at 1 wt % was added to this PHBH matrix. Crystallization behaviors under nonisothermal conditions were characterized using differential scanning calorimetry (DSC), while those under isothermal conditions were characterized using DSC and polarized optical microscopy. The results suggested that both cellulose and wood flour addition enhanced crystallization of the PHBH containing 5.6 mol % of 3HH (i.e., increased crystallization peak temperature and degree of crystallinity under the nonisothermal conditions, as well as decreased crystallization half time under the isothermal conditions). Of the two flours, wood flour was found to have greater effects, due to its higher crystal nucleating ability. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43600.