It was the purpose of present study to develop a new thermoseed for heating deep-seated tumors and assessment of the effect of magnetic control on thermoseeds. Aqueous suspension of iron micro spheres (Ferropolysaccharide) was injected directly into the VX-2 hepatoma and heated with 1.2 MHz inductive radiofrequency unit. Aqueous thermoseed suspension was delivered to the tumor by simple percutaneous injection. The limitation of the thermoseed heating method is the positional change of thermoseed particles in the tumor after implantation. The thermoseed particles could enter the systemic blood circulation and cause a severe embolization of a critical organ. To minimize this limitation, we have used the magnetic control after loading the thermoseed in the tumor. When ferropolysaccharides were exposed to a strong magnetic field, they magnetized and subsequently exerted a magnetic force on each other, forming larger aggregates of particles. The size of aggregated particles were too big to enter the systemic blood circulation. Thus, unlike other thermoseed method, we hold the thermoseed particles stationary in the tumor. The temperature of the injected site and immediate vicinity elevated by 4-5degree C. The temperature of the surrounding normal hepatic tissue elevated by 1-2degree C only. The heating effect within the tumor was variable depending on the density of ferromagnetic aqueous suspension. Our results suggest that inductive heating of tumor injected with ferropolysaccharide solution offers the possibility of effective heat delivery to the defined tumor volume, which is difficult to heat with other heating devices.