Bacterially synthesized poly(3-hydroxybutyrate-co-3-hydroxypropionate) [P(3HB-co-3HP)] samples were compositionally fractionated using solvent/nonsolvent fractionation techniques. Binary blends between these compositionally fractionated P(3HB-co-3HP) samples, with much narrower comonomer compositional distribution, were prepared, and their thermal and crystallization behavior were studied using differential scanning calorimeter (DSC) and polarized optical microscopy. It was found that, in the amorphous phase of these blends, the miscibility mainly depends on the difference of 3HP contents (or 3HB contents) between the two components of the blend. When the difference of 3HP content between two copolyesters is less than 30-40 mol %, they become miscible in the amorphous phase. The type of crystal lattice, miscibility, and difference of the spherulite growth rate between the two components of the blend appear to be the important factors controlling the crystalline phase behavior. The results are compared with those of unfractionated P(SHB-co-3HP)s and the effects of comonomer composition and its distribution on the properties of unfractionated copolymers are discussed.