The grafting of pegylated dendrons on 9(2) nm and 39(5) nm iron oxide nanoparticles in water, through a phosphonate group as coupling agent has been successfully achieved and its mechanism investigated, with a view to produce biocompatible magnetic nano-objects for biomedical applications. Grafting has been demonstrated to occur by interaction of negatively charged phosphonate groups with positively charged groups and hydroxyl at the iron oxide surface. The isoelectric point of the suspension of dendronized iron oxide nanoparticles is shifted towards lower pH as the amount of dendron increases. It reaches 4.7 for the higher grafting rate and for both particle size. Thus, the grafting of molecules using a phosphonate group allows stabilizing electrostatically the suspensions at physiological pH, a prerequisite for biomedical applications. Moreover the grafting step has been shown to preserve the magnetic properties of iron oxide nanoparticles due to super-super exchange interactions through the phosphonate group.