The article presents the results of research on the effect of magnetic hyperthermia performed on agar gel samples containing magnetite nanoparticles coated with dextran polymers for different molar weight M (150 kDa, 70 kDa, and 40 kDa). Regardless of the difference in molar dextran weights, these samples differed in a mass concentration of nanoparticles in the ferrogel C₀ (1.602 mg/cm³, 2.506 mg/cm³, 3.311 mg/cm³, and 4.218 mg/cm³). In the case of the highest magnetic field value H (20 kA·m⁻¹), the specific loss power SPL reaches 70 W·g⁻¹ for nanoparticles with 150 kDa dextran at a concentration of nanoparticles C₀ = 1.602 mg/cm³. An oscillating magnetic field with an amplitude up to 20 kA·m⁻¹ and a frequency of 357 kHz was used in the study.