The objective of this paper is to find sizing methodologies of pipe wall thinning using Electro Magnetic Acoustic Transducer (EMAT). First, SUS304 test specimens fabricated by an artificial corrosion on the back surface are provided and measurements are made by scanning the transducer on the front surface. Secondly, in order to clarify the propagation of an ultra-sound wave inside sample materials, simulation experiments are implemented using our developed numerical code. A sizing methodology for detecting and characterizing corrosion geometries is proposed based on time of flight diffraction method (TOFD). We report the method proposed is effectively examined through various kinds of test specimens.