A countercurrent blood vessel network (CBVN) model for calculating tissue temperatures has been developed for studying optimized hyperthermia cancer treatment. This type of model represents a more fundamental approach to modeling temperatures in tissue than do the generally used approximate equations such as the Pennes' bio-heat transfer equation (BHTE) or effective thermal conductivity equations. The 3-D temperature distributions are obtained by solving the conduction equation in the tissue and the convective energy equation with specified Nusselt number in the vessels. This study uses an optimization scheme to investigate the impact of thermally significant blood vessels during hyperthermia cancer treatment. The optimization scheme used here is adjusting power based on the local temperature in the treated region in an attempt to reach the ideal therapeutic temperature of 43℃. The scheme can be used (or adapted) in a non-invasive power supply application such as high-intensity focused ultrasound (HIFU). Results show that first, a large amount of thermal absorbed power is focused on the locations near (or in) blood vessels and/or dense vessels in the treated tumor region during hyperthermia treatment as mass flow rates contribute cooling effects from vessels to tissues. Second, veins also play a significant role of affecting temperature distributions in the treated region and need to be taken into consideration.