Eddy current testing (ECT) has been widely used for the non-destructive testing of metal structures. ECT has high capability of detecting surface breaking cracks and length sizing in nuclear power plants. However, it is known there is the error in the evaluation of the depth of the crack. Martensitic phase occurs due to the stress of crack in the vicinity of the crack and oxide layer of crack surface can affect the evaluation of crack depth. In this study, we assess the influence that impact these factors have on the sizing depth. Specifically, fatigue crack of different depths is introduced on the specimen of SUS316 by the four-point bending and an oxide film is progressed by heat treatment to evaluate the effect of the sizing. Further, martensitic phase in the vicinity of cracks is removed by heat treatment for reverse magnetic martensite to evaluate the effect on the sizing.