A blend polymer composition includes a styrene-based polymer, a non-styrene-based polymer, and a compatibilizer having a block copolymer. The block copolymer includes: a first block including a first monomer unit having an epoxy group, and a second monomer unit selected from an acrylate-based monomer unit, a methacrylate-based monomer unit, and a combination thereof; and a second block including a styrene-based monomer unit. The block copolymer has an epoxy equivalent ranging from 1000 g/eq to 20000 g/eq and a molecular weight distribution index ranging from 1.0 to 2.0.