Abstract-This paper presents a novel method for microwave breast cancer detection using a parallel-plate waveguide probe. The method is based on detecting the dielectric contrast between a malignant tumor and its surrounding tissues. Our analysis and simulations indicate that scattered signals from a tumor (modelled as a lossy dielectric sphere with higher dielectric constant than the surrounding tissues) received in the form of S parameter S 11 have resonating characteristics in the frequency range of 1 to 7 GHz. A frequency scan of the resonant scattered signals provides data of the presence and location of the tumor. Through numerical examples, the effectiveness of the proposed methodology to detect breast tumors of different sizes, embedded at different depths and to distinguish a tumor from clutter items is demonstrated.