We developed a new method using ultrasound-assisted degradation under alkaline condition for preparation of solution squid ink melanin.The squid ink melanin was mixed with NaOH solvent in brown glass tubes.The melanin-solvent suspension was treated by ultrasound for 1 h and then divided into three different molecular weight(Mw)fractions by membrane separation(below 10 ku,among 10-50 ku and over 50 ku).The result showed that the mean diameter of the squid ink melanin was decreased from 17.34 μm.to 1.467 μm after ultrasound process.UV,IR and solid¹³C NMR spectra indicated that the basic structure of melanin fraction was still retained after ultrasound process.The fraction with Mw below 10 ku showed more severe degree of oxidation on active groups(DHI and DHICA)than those above 10 ku.Further in vitro analysis showed soluble melanin fractions obtained in 0.5 and 1 mol/L NaOH,with Mw above 10 ku which had greater characteristic absorption peak of -OH and -NH structure exhibited much higher antioxidant potency.The IC₅₀ of these fractions(among 19-80 μg)on scavenging O^-₂· is more efficient than carnosine(IC₅₀=355 μg/mL.),a commercialized antioxidant.And they(IC₅₀ among 115-180 μg/mL)are as efficient as carnosine(IC₅₀ =110 μg/mL)on scavenging ·OH in free radical scavenge assay.In conclusion,our results showed that soluble squid melanin is a natural pigment and a new free radical scavenger with a promising prospect.The excellent solubility can increase absorption and enhance utilization rate in vivo,providing a potential prospect for squid ink melanin product as a natural antioxidant.