Water in oil(w/o) microemulsion had been used to prepare magnetic iron oxides nanoparticles with different microemulsion number density of Fe2+ and OH-. The as-synthesized magnetic iron oxides nanoparticles had been characterized by XRD, TEM, BET and AGM. The XRD result showed that the crystallite size of the samples was in the range of 3-8nm in diameter whereas the mean particle size obtained from TEM was in the range 2-6nm. For the least upper bound magnetic, size of these particles was at the range of 1-7nm. Due to the agglomeration, BET showed different result. The trend of the particles size versus the microemulsion number density of Fe2+ and OH- was the same. This showed that the particle size decreases when the microemulsion number density decreases and vice versa. By comparison of the as-synthesized magnetic iron oxides nanoparticles produced via w/o microemulsion and Massart's procedure, it was observed that the properties of both samples are similar. The only difference was on the particle size, which was 10.5nm and 4.06nm respectively. This proved that the w/o microemulsion was actually a compatible process if compare with Massart's procedure.