Ferromagnet has thermal effect in alternating magnetic field (AMF). Magnetically mediating hyperthermia is to localize magnetic substance within tumor tissue under AMF to promote the targeting ability and heat distribution of hyperthermia. This study was to develop a new method of showing the thermal effect of iron oxide Fe3O4 nanoparticles in AMF in vitro.Melted polyethelene glycol (PEG) was dropped on a slide to form crystal monolayer after cooling, and was covered with a bipartite blood coverslip. Certain amount of Fe3O4 powder (average diameter, 10 nm) was added in the gap of the blood coverslip to form a strip wandering on the slide with the width of 0.5-1.0 mm. This apparatus, named analog vessel bed, was then alternated in a 4.6 mT AMF at 100-250 kHz for 15 min. The changes of PEG crystal images were observed under micropolariscope. The thermal range of the heated Fe3O4 powder was affirmed according to melting status and melting point value of PEG crystal.In AMF, 5 mg of Fe3O4 could rise the temperature to above 45 degrees C-50 degrees C, which exceeded the required hyperthermic temperature 43 degree C; 15 mg of Fe3O4 could rise the temperature to no more than 59 degrees C-61 degrees C; while 25 mg of Fe3O4 could be heated up to 59 degrees C-61 degrees C. The PEG melting area was enlarged with the increasing Fe3O4 quantity or decreasing PEG melting point.The analog vessel bed apparatus could display the thermal effect of Fe3O4 nanoparticles in AMF, and would be helpful for further studies on effect of magnetically mediated hyperthermia on cancer cells.