Three kinds of non-stoichiometric ferrite nanospheres (including Co(II), Ni(II), Zn(II)-ferrite) with uniform and tunable size have been successfully synthesized by a simple one-step solvothermal reaction using a mixture of ethylene glycol (EG) and H₂O as the solvent without the addition of any surfactants or templates. Based on SEM and TEM characterization, all three kinds of products were determined to be single crystalline nanospheres built from numerous primary nanoparticles. The size of the different ferrite nanospheres prepared with same conditions differs slightly. For the same ferrite, the size of the nanospheres can be adjusted in a certain range by changing volume ratio of ethylene glycol and H₂O in the solvent. Three nanosphere samples were selected to investigate the gas sensing performance and magnetic properties. As gas sensing materials, all the three types of ferrites exhibited n-type gas-sensing behavior on the exposure to reducing gas atmospheres and the Co(II)-ferrite nanospheres exhibited high gas sensitivity and stability toward n-butanol. Magnetic measurements revealed that the Co(II)-ferrite and Ni(II)-ferrite nanospheres exhibited ferromagnetic property while the Zn(II)-ferrite nanospheres showed superparamagnetic behavior. The values of saturation magnetization (M_s) of the three samples follow the sequence of Zn(II)-ferrite (S3) > Co(II)-ferrite (S1) > Ni(II)-ferrite (S2) due to the difference of the magnetic moment of M(II) cations and their distribution in the spinel lattice.