The skeleton, swim bladder, muscle and so on are the basic structure of fish body, which is also the basis of studies on the classification and internal structure of fish. In recent years, many scholars at home and abroad studied the fish body internal skeleton structure for a variety of fish, but there was few exploration to the skeleton of tunas. X-ray computed tomography (CT) uses the characteristics of X ray that can penetrate the organism and the internal structure imaging of the organism can be built. X-ray CT has the higher resolution for the density of the organ. X-ray CT can display the large density difference between the soft tissue and skeleton structure clearly, it can also display the small density difference between soft tissue structures and organs. Because CT imaging is a kind of digital imaging, the image data of CT can be processed in different methods by using computer software, including three-dimensional display, etc., and can be used for further in-depth study. In this paper, the body of yellowfin tuna (Thunnus albacares) was scanned by X-ray CT, the layer thickness is 1 mm, and the study method and results can be used to provide the reference for the further studies on ichthyology, fish pathology and fish acoustic target strength. The T. albacares was provided by the Chinese pelagic fishery enterprise and was caught from the Kiribati waters in the western and central Pacific Ocean. The fork length, body wide, body height, and body weight of this fish were 66.0 cm, 13.6 cm, 16.5 cm, and 5.75 kg, respectively. Before the measurement, the fish was taken out from the cold storage 24 hs ahead of time. The fish was put into a CT scan system in the hospital. The model of the CT scan system was TOSHIBA Aquilion. The CT data were input into the medical imaging software Mimics (MIS_Research_21.0). The software can be used to identify the CT value (unit: HU) of skeleton, swim bladder, and muscles. The CT threshold value of the skeleton, swim bladder, and muscles were set to 126-1 500, –1015- –985, and 50-110 HU, respectively. The software automatically extracted CT values within the threshold range of skeleton, swim bladder, and muscles. The 3D images of skeleton, swim bladder, muscles, the body internal structure and the relative position of the swim bladder in the body internal structure of T. albacares were reconstructed and their corresponding volumes and surface areas etc. were measured. The results showed that: ① the clear images of T. albacares skeleton, muscles and swim bladder etc. can be obtained by X-ray CT scan; ② mimics, a medical imaging software, can be used to calculate the volumes and surface areas of skeleton, swim bladder and muscle of T. albacares. The volume of skeleton, swim bladder and muscle of T. albacares was accounting for 7.76%, 0.88% and 91.36% of the fish body volume, respectively and the surface area of skeleton, swim bladder and muscle of T. albacares was accounting for 29.52%, 0.74% and 69.74% of body surface area, respectively. This study suggested that: ① X-ray CT scanning is the low injury, most convenient and intuitive method for imaging fish skeleton, swim bladder and muscle; ② mimics, a medical imaging software, can be used to reconstruct the fish body and calculate the volumes and surface areas of fish skeleton, swim bladder and muscle; ③ X-ray CT scanning can be used for the in-depth study of ichthyology, fish pathology and fish acoustic target strength.