Abstract: In this paper, the technique of detecting leukemia and monitoring its treatment by microwave is presented. Two types of microwave source with operating frequency 800 MHz, i.e., line source and shorter antenna are considered. The finite-difference time-domain(FDTD) method is used to investigate the scattering field as the functions of the pathological degree of leukemia. The analysis shows that the better distinguishability and detective sensitivity can be obtained when shorter antenna is employed as microwave source.