Ultrafine NiFe2O4 particles with spinel structure were prepared in the system of water-in-oil microemulsion of water/Triton X–100/n-hexanol/cyclohexane. The effects of calcining temperature and the other operation conditions were systematically discussed based on the techniques of SEM, XRD, VSM and so on. The result showed that NiFe2O4 nanoparticles, with a narrow size distribution(30-70nm), could be obtained by microemulsion method. The crystal grain diameter of the ultrafine NiFe2O4 particles could be enhanced with the improvement of the calcining temperature, the molar ratio of water to surfactant (w=[H2O]/ [surfactant]) and the concentration of Fe3+(or Ni2+), respectively. The VSM plots showed that the ferrimagnetic behavior was expected for this type of magnetic material. The XRD patterns and DTA-TG curves revealed that the optimal calcining temperature of NiFe2O4 sample was at around 500°C. The relation between the average particle size (D) and the value of w could be expressed as D=13.911+0.2628w. Moreover, it was indicated that low concentration of Fe3+(or Ni2+) was beneficial to the formation of small and regular NiFe2O4 particles.