Abstract: A kind of cellulose magnetic nanoparticle with a core / shell structure has been prepared by ultrasonic irradiation. Cellulose acts as the shell while Fe3O4 magnetic nanoparticles take the role as the core. Magnetic force microscopy（MFM）with atomic force microscopy（AFM）measurement showed that the size of the magnetic nanoparticles is about 30-50 nm in diameter，while the Fe3O4 core is about 20-30 nm. FT-IR，XRD and MFM was used to provide the chemical and magnetic information of the nanoparticles. The MFM image showed that the nanoparticles separate very well with each other，indicating the cellulose shell produces a good prevention from the aggregation of the Fe3O4 particles. MFM studies also showed two magnetic nanoparticles can form particle-pairs，indicating a weak magneto-dipole interaction between magnetic nanoparticles. It is also found that the average sizes of magnetic nanoparticles have relation to the power of ultrasonic irradiation，and the possible mechanism is discussed.