Myoglobin (Mb) was isolated from the dark muscle of the milkfish (Chanos chanos) by Sephadex G-75 gel filtration chromatography. The molecular weight of milkfish Mb is 15, 900 Da. The tristimulus color values (L, a and b) of Mb solutions changed significantly depending upon the form of Mb. With the formation of metmyoglobin (metMb), the L value increased gradually with the increase in the percentage of metMb, while a values decreased and Mb aggregation gradually increased along with incubation time. The autoxidation of milkfish Mb proceeded as a first-order reaction, and the autoxidation rate constant increased with decreasing pH in a range of 5.5–7.0. The free energy for unfolding (Δ GD) of Mb at acidic pH was smaller than at neutral pH, with values of 5.8 kcal/mol at pH 6.63 and 6.0 kcal/mol at pH 7.05. A higher rate of autoxidation and lower values of free energy were observed at acidic pH than at neutral pH.