Ultra-wideband (UWB) microwave imaging is a very promising method for the early breast cancer detection physically based on the large contrast of electromagnetic parameters between the malignant tumor and the normal breast tissues. In this study, the tumor detection in a two dimensional breast model with dispersive characteristic properties of the breast organisms is carried out numerically by the finite difference time domain method (FDTD). The Perfect matched layer absorption boundary condition (PML ABC) is adopted in the computation. The influences from the skin, breast gland and the chest wall to the breast cancer detection by UWB imaging are investigated. The detection for an assumed 6 mm early stage tumor which located behind the gland is performed. The reconstructed image shows that the tumor could be detected correctly and clearly through the appropriate signal processes.