In the piping system of power plants, the pipe wall thinning caused by flow accelerated corrosion (FAC), liquid droplet impingement (LDI) erosion, and cavitation erosion (C/E), are very serious problem because they lead to serious damage and destruction of the piping system [1]-[6]. In this study, pipe wall thinning caused by FAC in the downstream of an orifice nozzle (flow meter) was examined. Experimental and numerical analyses were carried out to clarify the characteristics of FAC, generation mechanism, and to propose of a new governing parameter, the prediction of the thinning, and reduction method. The corrosion pattern on the pipe wall was also examined by an experimental simulation [7]. This simulation clarified that the occurrence of thinning mainly depend on the amount of pressure fluctuation p' on the pipe wall. It was also found that the wall thinning rate can be estimated using p' and that the suppression of p' can be realized by replacing the orifice nozzle with a tapered one having an angle to the upstream or using a downstream pipe with a smaller diameter than that for the orifice.