In this paper, study on evaluation of thickness reduction in a thin plate with guided waves is presented. Ultrasonic guided wave techniques have been widely studied and successfully applied to various non-destructive tests with the advantage of long range inspection. In addition to application of guided waves to NDT, non-contact methods for ultrasonic wave generation and detection have become very useful and well combined with guided wave techniques due to their capability of ultrasonic wave generation and reception in surface of high temperature or on rough surface. An advanced non-contact technique for detection of thickness reduction simulating hidden corrosion in thin plates using guided waves is proposed. The proposed approach uses EMAT(Electro-Magnetic Acoustic Transducer) for the non-contact generation and detection of guided waves in aluminum plates. Interesting features of the dispersive behavior in selected wave modes are used to detect plate thinning. The experimental results show that the mode cutoff measurements provide a qualitative measurement of thinning detects and change in the mode group velocity can be used as quantitative parameter of thinning depth measurement.