The effects of the affinity of myoglobin for oxygen on meat discoloration were investigated. First, we showed that 10 mmol/L tartrate significantly decreased the deoxygenation rate in vacuum-packed beef, although it did not reduce mitochondrial oxygen consumption activity. Second, the myoglobin affinity was evaluated with or without 1 mg/mL mitochondria and/or 10 mmol/L tartrate. The oxygen concentration that gives 50 % of the deoxygenation ratio was evaluated as myoglobin affinity. The affinity in the "with mitochondria" condition (0.370 ± 0.006 %) was significantly lower than that of "myoglobin" (0.312 ± 0.019 %), "with tartrate" (0.292 ± 0.009 %), and "with mitochondria and tartrate" (0.287 ± 0.022 %). Furthermore, the effect of 10 mmol/L tartrate on the myoglobin oxidation rate in beef was observed at 0.18-4.2 % oxygen. The addition of tartrate inhibited oxidation at lower concentrations but promoted oxidation at higher concentrations, with 1 % being the divergent concentration. Using tartrate, we demonstrated that myoglobin affinity is a determining factor for the oxygen concentrations that promote beef discoloration.