Medical robotic system enable precise and safe treatment for patients, especially in the area of urology. In this paper, we propose a robotic palpation system to detect prostate cancers. These cancers usually arise in the posterior of the prostate and as they are firmer than normal tissues, can be detected by digital rectal examination (DRE). This system is designed to induce a controllable deformation to tissues as well as to measure the force response of tissues. To validate the system performance, ex vivo experiments on human organs were conducted to determine the differences between the mechanical properties of normal and cancerous tissues. The tissue properties were characterized using experimental results. In addition, we propose the numerical maps expected to be used to diagnose prostate cancer by using the obtained elastic moduli of normal and cancer tissues. The results indicate that this system could detect differences of mechanical properties between normal and cancerous prostrate tissues.