The effect of ultrasonic thawing (0, 160, 280, 400 W) on water-holding capacity (WHC), physicochemical properties and structure of tuna myofibrillar proteins was investigated.Thawing time was shown to decrease and thawing loss to increase significantly (P < 0.05) as power increased (160-400 W), whereas there was no significant difference (P > 0.05) in cooking loss. Changes in T relaxation time were investigated using low-field nuclear magnetic resonance. Ultrasonic thawing could significantly (P < 0.05) improve the immobilised water content compared to the control (0 W). surface hydrophobicity decreased significantly and then increased significantly (P < 0.05), whereas there was no significant difference (P > 0.05) in the reactive sulfhydryl content as power was increased. Tuna thawed at 280 W suffered fewer negative effects on its microstructure. Roman spectral date showed that the α-helix changed to a random coil and β-turn as power was increased (up to 400 W).The application of ultrasonic thawing at a specified power was showed to be a beneficial process when used in the seafood industry, but application of excessive power resulted in lower WHC and structural changes to myofibrillar proteins. © 2019 Society of Chemical Industry.