Breast cancer is one of the most common and life-threatening diseases among women. Hence, early diagnosis is important to detect cancer tissue. But the common techniques used for breast cancer detection have limitations. So, microwave imaging (MWI) techniques are developed. MWI techniques are rather interesting and promising. In this paper, a rectangular microstrip patch antenna (RMPA) was designed and modified for breast cancer detection. And a breast phantom was modeled with tumor and without tumor using Computer Simulation Technology Microwave Studio (CST-MWS). Once the breast tumor is embedded within, the results showed that the current density, the magnetic field, the electric field, and the power loss density have been increased by 30 A/m2, 10 A/m, 1800 V/m, and 100000 W/m3, respectively. These results show that the described RMPA is capable to detect the tumor in the breast with better characteristics than the other available designs in the literature.