REBCO coated conductors has a layer structure composed of Cu, Ag, GdBCO, MgO and Hastelloy substrate. Although the REBCO conductors enables high field magnet designs, their critical current properties are sensitive to mechanical stress. Especially, the thermal stresses in the cooling process of the REBCO magnets may cause the delamination of the layer structure of the REBCO conductor. This problem leads to the defectiveness of the superconducting magnets. The objective of this work is to establish the delamination diagnostic method using ultrasonic waves and to estimate the superconducting properties based on the diagnostic results. Since the mechanical stresses between the layers can not be measured directly, the authors try to investigate the relationship between the transfer characteristics of the ultrasonic waves through the layer structure and the critical current properties by using acoustic emission (AE) sensors. Two AE sensors are set on both sides of the REBCO conductor surface. The thickness of the REBCO conductor is 0.2 mm. The one sensor is used as a transmitter which provides 1-μs width of the pulsed signal with 1-ms interval. The other sensor is used as a receiver which receives the AE signals through the REBCO conductor. In order to delaminate the layer structure, the REBCO sample is partly applied by heat stresses for 5 minutes using a soldering iron. The transfer characteristics of the AE signals are measured in room temperature and evaluated their power spectrum. The resonance frequency is around 366 kHz. From the results, the power spectrum around the resonance frequency at the delamination point is reduced to 40% of that without the delamination point, which fact shows the feasibility of the delamination diagnostic method using AE sensors. In this work, the authors will compare the diagnostic results to the critical current properties with liquid nitrogen cooling.