Determination of light absorption distribution in the prostate tissue irradiated by diffusing light source is important for the treatment planning. In this paper, a three-dimensional (3D) optical model of human prostate is developed, and the light absorption distribution in the prostate tissue is estimated by Monte Carlo simulation method. Light distribution patterns including 3D distributions in the tissue model irradiated by two diffusing light sources are obtained and compared. Also, the impacts of length and energy of cylinder diffusing light source on the irradiance volume are demonstrated. Those results will be significant for the nondestructive qualitative assessments of photodosimetry in biomedical phototherapy.