A method to evaluate the shape and size of a small 3-D crack by microwaves was demonstrated. A component named interference waveform , which is based on the amplitude of the reflection coefficient measured at two different frequencies , was obtained. On the other hand, based on the interference model, a corresponding interference waveform, which can be calculated from the assumed shape and size of a 3-D crack, was also introduced. By compar ing these two interference waveforms, the evaluation of the shape and size of a 3-D crack was carried out. The shape and size of the crack were estimated well by using the proposed method.