The bulk crystallization behavior of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] were studied using polarized light microscopy, differential scanning calorimetry, and wide- and small-angle X-ray diffraction. Under the isothermal crystallization condition, the linear growth rates of the P(3HB-co-4HB) spherulites were measured at various temperatures and the influence of mutual disturbance for both comonomer components was clarified. The long period of P(3HB-co-4HB) increased much less with an increase in the crystallization temperature, compared to that of P(3HB). These results imply that the P(3HB) crystal excluded the 4HB component when, for example, the content of 4HB component in the copolymer was 5 mol%.