A new device that can measure the in vivo optical properties of internal organs such as the prostate was designed and a calibration procedure for clinical use was developed. It is composed of two optical fibers running in parallel. Therefore, the size of the probe is small enough to be placed under direct vision transurethrally in the prostate in a minimally invasive manner. This device is applicable for both single and multiple wavelength analyses and was used to measure the optical properties of both canine and human prostates. The preliminary results show that the optical penetration depth might be considerably different between canine and human prostates. Penetration depths ranging from 0.5 - 0.7 cm for canine prostate and 0.2 - 0.3 cm for human prostate over the wavelength of 650 - 750 nm were measured in vivo.