We discuss details of thermal therapy of brain tumors carried out with ultrasound interstitial instrumentation. A case of heating with four applicators placed in the tumor region is discussed. The blood flow influence on temperature pattern is examined using finite element analysis. This influence is illustrated for an arteriole splitting into 1.1- and 0.6-mm diameter branches passing near the tumor. Two blood flow values, 45 and 150 ml/min/(100 g), were investigated. The tumor of various structures is modeled: it is considered as a tissue of uniform physical properties or with its about 30% volume necrotic center of different physical properties. We found that the temperature pattern is strongly modified by both the tumor structure and the blood flow. It was also realized that the branches role was quite complex: while the smaller diameter branch could help in maintaining more uniform temperature, the larger diameter branch caused significant drop of temperature.