Magnetite hollow submicrospheres and mesoporous nanoparticles have been synthesized by a solvothermal approach with assistant of hexamethylenetetramine (HMT) and poly(vinyl pyrrolidone) (PVP). Experimental results showed that the morphology of magnetite was transformed from hollow to solid submicrospheres with increasing amount of HMT. Moreover, without addition of PVP or appropriate addition of external water, mesoporous nanoparticles were obtained. A probable gaseous bubble template mechanism was proposed for the formation of magnetite hollow and mesoporous nanostructures based on experimental observations. Magnetic measurement results revealed that all of the samples were ferromagnetic at room temperature.