The effect of a theological factor on a temperature field in a heterogeneous tissue in its SHF heating is considered. A microvascular system is presented in the form of quasiporous media and is replaced by an equivalent tube. The relation is found between relative change in perfusion, the parameter of blood viscoplasticity and hydraulic diameter of an equivalent tube. Perfusion is found to be proportional to the cube of a hydraulic diameter of this tube and conversely proportional to the value of the parameter of viscoplasticity. Their possible values and the range of variation with local hyperthermia (LH) are estimated. II is shown that the values of the parameter of viscoplasticity within the range 1-0.06 correspond to small shear rate (5-100 s(-1)). The changes in the hydraulic diameter in a normal tissue in LH may attain 20-80%, the changes in the parameter of viscoplasticity may vary by 2.5 and more limes. It is shown on the example of frequencies 2450 and 915 MHz that the theological factor of blood flow exerts a very substantial effect on a temperature field in tissue, especially in the presence of a slightly perfused tumour in it.