This experiment aimed to study the quality changes of tuna at low temperatures. Changes in metmyoglobin content, thiobarbituric acid-reactive substances (TBARs) value, the freshness indicator K value and pH of o-toro from Thunnus atlanticus, Thunnus obesus and Tetrapturus audax during storage at −18 and −30 ℃ were examined. The results showed that with the extension of storage time, the o-toro of all three tuna species exhibited an increase in pH, K value, TBARs value and metmyoglobin content, and all these parameters changed significantly faster at −18 ℃ than at −30 ℃. Compared with TBARs value and K value, metmyoglobin content was a better indicator of the eating quality of raw tuna, reaching a level higher than 30% after storage for 14 days at −18 ℃. In addition, the color of tuna changed from red to reddish brown, suggesting significant deterioration of its sensory quality. But the storage period at −30 ℃ was up to 28 days.