The self-assembly of binary blends composed of AB diblock and ABC triblock copolymers is investigated using selfconsistent field theory. Specifically, the miscibility of the binary mixture is studied by tuning the concentration ϕ 2 of AB diblock and the length ratio γ between AB diblock and ABC triblock. It is predicted that the sphere-forming AB diblock and binary sphereforming ABC triblock copolymers can be compatible by coassembling into new spherical phases, even when ϕ 2 is close to 50%. For relatively low γ (0.48 < γ < 1.13), the two different copolymers are accommodated into an asymmetric binary spherical phase, Li 3 Bi, where the A-sphere coformed by two A-blocks with similar lengths is close to the C-sphere in size. When γ is increased to larger than 1.45, the two A-blocks have considerably different lengths, and they form a core−shell distribution within the A-sphere. The core−shell distribution effectively enlarges the A-sphere. The two factors where the A-sphere is larger than the C-sphere and the volume fraction of A-blocks is larger than that of the C-block lead to the formation of NaCl phase with symmetric coordination number. Our theoretical results demonstrate that blending simple block copolymers with proper length ratio and concentration provides an efficient route for the fabrication of new binary mesocrystals.