We have fabricated similar to 143 nm Fe3O4 thin films on Si(100) substrates at 450A degrees C and then annealed them at the same temperature for 30, 60 and 90 min under a vacuum of 10 (-aEuro parts per thousand 6) torr with pulsed laser deposition. We studied the effects on the structural and magnetic properties of Fe3O4 thin films. The films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). XRD studies showed pure single phase spinel cubic structure of Fe3O4 with a preferential  orientation, independent of substrate orientation at 90 min annealing. Higher magnetization was obtained up to 60 min annealing due to Fe phase but at 90 min, we obtained reduced magnetization of 335 emu/cc. This is attributed to the formation of antiphase boundaries between substrate and film.