Poly (β-hydroxybutyrate) and its copolymer poly (β-hydroxybutyrate-co-β-hydroxyvalerate) P (HB-co-HV) were treated with methyl- and ethylamines. Melting peak temperatures, number-average molecular weights (Mn) and long spacings (L) of treated samples decreased as the concentration of amines or the treating time increased. However, Mn and L almost levelled off at the optimum conditions, where amorphous regions at the surfaces of crystal lamellae were removed to isolate the core parts of single crystals. The level-off Mn values corresponding to the real thickness of the crystal core were estimated. The equilibrium melting point of PHB was ca. 200°C which was estimated from the plot of melting point Tm against L-1. The content of HV component in the crystalline region of P (HB-co-20mol% HV) evaluated by 1H-NMR was 18mol%, which was slightly smaller than the initial composition (20mol%). This implies that a small number of HV units was excluded from the HB crystals due to the bulky side group of the HV unit.