In this study, corrosion fatigue test of SAPH45 steel was performed by the use of plane behavior of base metal (BM) and heat affected zone (HAZ) of SAPH45. The main results obtained are as follows: 1) The more aspect ratio (b/t) of corner crack decreases, the more aspect ratio (b/a) takes greatly effect by corrosion. 2) The correlation between the stress intensity factor range ($Delta$k) and crack growth rate (da/dN) for weldment in seawater is given by Paris rule as follow: da/dN=C($Delta$K) super(m). Where m is constant, and the value is 3.82-3.84. 3) The accelerative factor ($alpha$) of BM and HAZ under seawater is about 1.1-1.9, and ($alpha$) of HAZ increases more and more under the low $Delta$K region. 4) HAZ is more susceptible to corrowion than BM because of potential of electrode (E sub(c)) of HAZ becomes more less noble potential than that of BM.