The guided ultrasonic wave has the advantage of diagnosis on a wide area within in a short time due to the long distance propagation characteristic. However, there are many difficulties in signal analysis due to the mode conversions in the reflection from the defect and boundaries. In the use of guided waves for structure monitoring, it is necessary to understand the relation between the propagation mode and the mode of variation according to the shape of the defect. In this study, the characteristics of induced ultrasonic mode conversion is analyzed in taper defects formed from the surface of an aluminum plate. The defect depths of the plate thickness are 20, 50, and 80% and the characteristics of the reflection and transmission modes are analyzed on various defect widths, depending on the angle of change of the tapered shape. The A0 and S0 modes were selected as the excitation mode of the guided waves, the transmission and reflection coefficient amplitudes are analyzed. It is confirmed that the wavelength of the excitation mode having a large influence on the amplitude of the transmission and reflection signals generated by the taper defects depend on the shape of the defect.