In nondestructive testing (NDT), ultrasonic testing (UT) can inspect internal defects of the material with a simple device. General UT is performed by point measurement. On the other hand, UT using guided waves that propagate long distance in pipe and plate structures is expected as an efficient inspection. However, there are multiple dispersive propagating modes in the same frequency band, and they are very complicated. For highly accurate inspection, it is important to determine the amplitude of each guided wave mode. Generally, the dispersion relation of the guided waves mode is determined by steady-state analysis, so the amplitude ratio for each mode cannot be determined. This study deals with Lamb waves that propagate plate among guided waves. The steady- state amplitude of each mode was obtained by transient analysis of the growth process of each mode.