The stress,strain and deformation after the hook in the stress play a decisive role on the mechanical properties and structural stability of the hook.The pros and cons of the mechanical properties of the hook directly affect the capture effectiveness of the target species.It is beneficial to improving the hook design and material selection to understand the mechanical properties (eg.tension and deformation) of tuna longline hook.There are some advantages for the circle hooks,such as reducing the sea turtle by catch rate,high survival rate after capture release,low escaping rate,etc..There is a trend that circle hook replaces the ring hook and J hook.The deformation or fracture of hook will make longline fishing invalid.Therefore,the hook should have enough stiffness and strength when the hook is designed and the material is determined.Mechanical analyses are carried out on two kinds of typical hooks (ring hook and circle hook) using finite element analysis method (FEM) by ANSYS.The distribution of equivalent stress,strain and deformation of hook are obtained,and the possible failure condition and the failure type are also analyzed.Intensity examination and optimization design are made on hookstructure based on material mechanics theory,compare with hook's data obtained in the tensile test.Results showed:(1) With the same specifications,ring hook'stensile strength is higher than that of circle hook by 15.80%-29.40%,and the max. deformation rate in X and Y axis of ring hook is smaller than that of circle hook by 3.00% and 0.21% respectively;(2) Hook's main stress is undertaken by the bend of the hook (83.00%-86.00%),and there is a stress concentration in the hole of hook eye;(3) Flattening the bends surface (flattening rate is 15%),increasing the hook circular arc of the bending curvature appropriately (the radius increased from 14 mm to 16 mm),connection of hook eye and shank added with arc transition can avoid stress concentration and enhance the strength of the hook by 4.60%-10.80%.This study suggests that (1) Circle hook is the eco-friendly hook type;(2) Its tensile strength meets the requirement of the fishing;and (3) It can be used in the longlining.