In this research, corrosion fatigue tests using 490MPa TMCP steel were performed in synthetic seawater condition to investigate the corrosion fatigue crack propagation characteristics of TMCP steel plate which is often used for ships and offshore structures from its weldability and weight reduction purpose. In addition, relationships between boundary correlation factor F(α,β) and α(= 2a/W) of the fracture mechanics formula for the test specimen which was exerted by the pin load were calculated for the evaluation of da/dN-.K. To get the boundary correlation factor F(α,β), the calculation of J as a path-independent line integral was carried out. The solutions of J integral showed similar value in spite of the different path. Crack propagation tests for the same steel in air condition was carried out also for comparison. Finally, it was clear from the crack propagation test results that the propagation rate of TMCP steel plate in synthetic seawater condition is faster than that of in air condition at least twice.