In local hyperthermia it is important to leave the temperature of surrounding normal tissues essentially unchanged while increasing the temperature of tumors to a desired level. Soft heating1 with temperatureÃ¢â‚¬Â�sensitive magnetic materials is advantageous in heating deepÃ¢â‚¬Â�seated tumors because of the selfÃ¢â‚¬Â�limiting ability. In order for the method to become available for largeÃ¢â‚¬Â�scale tumors we have proposed to use temperatureÃ¢â‚¬Â�sensitive magnetic powders, in which fine metal rings are dispersed. As the magnetic flux distribution varies with the temperature of the powders, the fine rings immersed in the powders with low temperatures generate heat more than rings located in the powders with high temperatures. Furthermore, the transition from the ferromagnetic to the nonferromagnetic state becomes sharp effectively. As a result it is expected to make the temperatures of powders in a broad region uniform. Figure 1 shows the temperature dependence of the magnetic powders used in experiments. Figure 2 shows the relation between the attained temperature of the powders and the ring resistances, comparing them in the case of iron powder. The figure reveals that it exists appropriate combination between the ring resistance, and the magnetic permeability of the powders.