This work is focused on the functionalization of flax fibers aiming to improve their flame retardancy by grafting a phosphorus-based flame retardant (FR), dimethyl(methacryloxy) methyl phosphonate (MAPC1). The pre-irradiation method was used, and the grafting process follows three main steps namely: the irradiation of flax fibers, the impregnation of irradiated fibers in FR solution and finally a washing step to remove unreacted monomer units and free oligomers and polymers chains not covalently bonded to the flax structure. The presence of radicals on the flax fibers after irradiation was 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. Different parameters were studied such as the radiation dose, the temperature and the duration of the grafting reaction and the monomer concentration. In particular, it was observed that MAPC1 was 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 leading to high charring and low flammable fibers. The assessments are based on the measurement of density and thermal conductivity. The variation of compressive strength in function of the characteristics (mean particle length) of rice straw particle are assessed and discussed. The investigation covers also the porosity and density. Tests are also carried out on agricultural by-products with a view to highlight their chemical, physical and structural proprieties. The results show that the use of large particles with low water absorption capacity induce lighter concretes with the density between 339 and 505 kg/m3 and lead to a high compressive strength with a high mechanical deformability. Furthermore, it appears that an increase in the average length of rice straw particle lead to decrease of thermal conductivity of bio-based concretes. It varies from 0.062 to 0.085 W/(m.K).