Pre-irradiation method was used to modify flax fabrics by grafting phosphorus compouds to improve their flame retardancy. A phosphorus-based flame retardant (FR) dimethyl(methacryloxy)methyl phosphonate (MAPC1) was used as unsaturated grafting agent. The grafting process follows three main steps. First, flax fibers were irradiated under electron beam at a dose ranging between 5 and 100 kGy, then the irradiated fabrics were impregnated by dipping them into an aqueous solution containing the MAPC1 monomer; finally a washing step of the treated fibers allows to remove unreacted monomers units and the free oligomers and polymers chains which are not covalently bonded to the flax structure. Different parameters have been studied such as radiation dose, temperature and duration of the grafting reaction and monomer concentration. In particular, it has been observed that MAPC1 is grafted in a homogeneous way into the bulk of the elementary flax fibers leading to high phosphorus rate of around 2 wt% for a dose 10 kGy and up to 7 wt% for a dose 100 kGy. The presence of radicals on the flax fibers after irradiation has been confirmed by Electron Paramagnetic Resonance (EPR). Grafting efficiency was assessed by infrared spectroscopy (FTIR) and quantified using Inductively Coupled Plasma (ICP-AES). The location of the grafted phosphorus polymer chains was assessed by Scanning Electron Microscope coupled with Energy Dispersive X-ray spectrometer (SEM-EDX) using phosphorus mapping of modified fibers. The effect of phosphorus grafting on thermal properties and fire behavior of flax fibers was studied using thermogravimetric analysis and pyrolysis combustion flow calorimetry.