A core-shell ensemble of bovine hemoglobin (Hb) and human serum albumin (HSA) is an artificial O2 carrier as a red blood cell substitute. This protein particle is created by covalent wrapping of a carbonyl Hb with HSAs: HbR-HSA 3 cluster, where HbR signifies the use of carbonyl Hb (relaxed (R) state conformation) as a starting material. The HbR-HSA 3 cluster exhibits high O2 affinity and low cooperativity. Analysis of the quaternary structure of the central HbR in the cluster revealed that its high O2 affinity is attributed to the physically immobile HbR nucleus. Circular dichroism and UV-vis absorption spectroscopy showed that the structure of deoxy HbR core closely resembles the R-state. The crystal structure of Lys-modified carbonyl HbR was superimposed on that of carbonyl Hb. These results imply that chemical modifications of the surface Lys groups and Cys-93(β) of the carbonyl Hb with cross-linking agent interfered in the quaternary structure movement from the R-state to tense (T) state. As expected, coupling of deoxy Hb (T-state) with HSAs yielded HbT-HSA 3 cluster having low O2 affinity. The mixing of HbR-HSA 3 and HbT-HSA 3 clusters conferred a tailor-made formulation of artificial O2 carrier with a desired O2 affinity ( P50).