An easy-to-use and environmentally friendly method is proposed to increase the fire retardancy of natural fibers. Hemp fibers were phosphorylated by the grafting of phytic acid, a renewable and environmentally friendly phosphorous flame retardant and urea in aqueous solution. The fibers were characterized by X-ray fluorescence, solid state 13C and 31P NMR analysis, pyrolysis combustion flow calorimetry and tensile testing. The swelling behavior of the fibers evaluated by optical microscopy observations showed that urea promotes swelling and consequently the grafting of P and N in the fiber. It has been also shown by a bleaching step that delignification makes the cellulose slightly more accessible for grafting. Non-flammable fibers were produced with relatively low P and N contents (> 0.5 % w/w and > 0.7 % w/w respectively). However, the phosphorylation process significantly decreased cellulose crystallinity and mechanical properties of the resulting fibers. For P = 0.9 %, a 30 % decrease in the tensile strength of hemp fibers was observed. The production of phytic pyrophosphate by thermal dehydration of phytic acid has been proposed from 31P NMR results.