The close-packed structures of spherical microdomains in polybutadiene-poly(ε-caprolactone) diblock copolymer (PB-PCL) / polybutadiene (PB) were investigated using small angle X-ray scattering with synchrotron radiation. Two kinds of homopolymers,PB1k (Mn = 900 g/mol) and PB2k (Mn = 1700 g/mol), were used. In the PB-PCL/PB1k blend sample with a weight fraction of added homopolymer (φPB1k) of 40 wt%, the hexagonal close-packed lattice (HCP) was formed, while face-centered cubic lattice (FCC) was observed in the sample with φPB1k ≥ 56. In PB-PCL/PB2k with φPB2k = 50 wt%, spheres arranged in non-equilibrium liquid-like packing. The HCP lattice structure appeared in the blend with φPB2k = 56 wt%, whereas the FCC lattice structure was observed for φPB2k = 67 wt%. It was found that the lattice changed from HCP to FCC with increasing amount of homopolymers. The concentration dependence of radius of spherical micelles (Rc) was also investigated. The size of R in PB-PCL/PB2k was larger than that in PB-PCL/PB1k irrespective of weight fraction of homopolymer. In addition, concentration dependence of Rc was almost constant in PB-PCL/PB2k, but R of PB-PCL/PB1k was decreased with increasing φPB1k.