The structure of the isomorphous crystals of P(HB-co-HV) in the PHB crystalline lattice is examined in detail, and a structural transition at ca. 10 mol % HV is found for the first time. By analyzing a series of P(HB-co-HV) samples prepared by the same treatment, the effect of the thermal history was removed from the results, and the composition dependence of the crystal formation and of the resultant crystalline structure was highlighted. The crystallinity, unit cell dimensions, and lamella thickness of P(HB-co-HV) were determined by X-ray analysis. The HV content in the crystalline phase was determined by high-resolution solid-state C-13 NMR spectroscopy. The thermal properties were studied by DSC. The crystallization kinetics was analyzed through the measurement of spherulite growth rate. It has been shown that the composition dependence of spherulite growth rate, crystallinity, lamella thickness, HV content in the crystalline phase, and melting temperature change discontinuously at ca. 10 mom % HV, indicating the occurrence of a structural transition at this composition. To explain these results, we have introduced two models of lamellar crystals containing counits, i.e., sandwich lamella and uniform lamella. We have concluded that P(HB-co-HV) with HV content below and above 10 mol % can be described by the sandwich lamella model the uniform lamella model, respectively.