Nanocrystalline Fe3O4 particles of controlled size were synthesized by a chemical co-precipitation method. X-ray diffraction patterns confirm the synthesis of single crystalline phase of Fe3O4 nanoparticles with the lattice parameter 8.409 A. The particle size is 11.5 nm and increases to 20 nm after annealing. TEM image shows that the sample contains well dispersed nanoparticles with an average particle size of the order of 20 nm. Magnetic measurements show the superparamagnetic nature of the sample that is also confirmed by EPR measurements. The saturation magnetization is 42 emu/g. The results are explained adequately.