Breast cancer (BC) is one of the most common diseases in women. The accurate detection for BC is very important in clinical practice. In this paper, in-vivo breast cancer in a subcutaneous xenograft mouse was studied with the continuous-wave terahertz (THz) reflection imaging system at 2.52THz. The high reflection region in THz images of in-vivo subcutaneous xenograft mouse corresponded well to the tumor area in visible. The difference value of 15% between tumor and normal tissues was observed. For the gold standard, the tumor areas at different depths from the surface of skin in mouse model were obtained by hematoxylin and