The purpose of this thesis is going to design a microstrip antenna that can detect the different tissue, according to this clue, we suppose that it can be used in medical application, for example to detect the breast tumor. Our research and guess are based on the different electrical properties of the tissue. Follow the conductivity and permittivity of different tissue, a simple 3D breast structure is going to be modelled, and a tumor tissue will be defined too. Four types of antenna have been designed and simulated in HFSS. The antenna with best performance will be manufactured, which is the rectangular microstrip antenna with gap, it works under 2.45Ghz resonate frequency, and with dimension 37.26mm×28.83mm. Test S21 of antenna in Vector Network Analyser with different biological tissue, in order to make a simple prove that the antenna can detect the different tissue. The result shows when we use the different tissue’s size and quantity, the S21 changes.