Cellulose/magnetic nanocomposites were prepared by an ultrasound-assisted in-situ composition using cellulose fibers as matrices. The effects of pretreatment and maturing method on the composite efficiency were comparatively studied. Firstly the effects of ultrasound wave and mercerization pretreatment on the composite efficiency were investigated. Then the influences of ultrasound-assisted maturing on the composite efficiency were also discussed. Finally the nanocomposites’ structures were investigated by means of SEM, AFM and X-ray diffraction and the magnetic properties of the nanocomposites were determined by superconducting quantum interference device (SQUID). Results showed that ultrasound wave pretreatment was more efficient for the in-situ composition compared to the mercerization pretreatment. The amount of magnetic particles (valued as mass fraction of Fe) for untreated, ultrasound wave treated and mercerization treated cellulose fibers were 6.70%, 7.67% and 3.14%, respectively. The mass fraction of Fe increased from 5.04% for the mechanical agitation-maturing prepared sample to 5.61% for the ultrasound-assisted maturing prepared sample. Both ultrasound pretreatment and ultrasound-assisted maturing can remarkably raise the composite efficiency and magnetic particles’ yields. This reveals that ultrasound wave assisted-maturing is an effective method to control both the size and the distribution of magnetic iron oxide within the composites. Cellulose/magnetic nano-composite fibers with ferrite particles in size between 10 nm to 100 nm were obtained. XRD analysis indicates that the iron oxide synthesized is mainly in the C phase of g-Fe2O3. SQUID results reveal that the ultrasound-assisted in-situ synthesized cellulose/magnetic nanocomposites are superparamagnetic.