Melting and crystallization of two types of polyhydroxyalkanoates (PHAs), poly (3-hydroxybutyrate) homopolymer (PHB) and poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) random copolymer (PHBHHx) were studied via in situ Fourier transform infrared spectroscopy (FTIR). The absorbance variances of the crystalline and the amorphous bands revealed that melting of PHB occurred at a narrow temperature range, while melting of PHBHHx copolymer occurred at a wide temperature range, and it was shown that the latter had a much wider distribution of lamellar thickness. These results were consistent with those from DSC. The alkyl groups reached close packing prior to crystallization in PHB and followed the crystallization in PHBHHx due to the longer branch propyl group. The band at 1230 cm21 was first assigned to the conformational band of the helical segments, and the absorbance ratio of the band at 1230 cm21 to the reference band at 1453 cm21 was used to quantitatively determine the crystallinity of PHAs after adjustment with DSC. Since further crystallization usually occurs in PHAs during heating and DSC could not reveal the original crystallinity in these samples, we suggest FTIR as an effective and nondestructive tool to evaluate crystallinity of PHAs.