A biopolymer-based catalyst with an alkaline moiety was prepared by radiation induced grafting (RIG) of glycidylmethaacrylate (GMA) onto flax fibres followed by functionalisation with diethyl amine (DEA) and treatment with NaOH. The density of amine loading was tuned by optimisation of the reaction parameters including DEA concentration, reaction temperature, and time using the Box�Behnken Design (BBD) of Response Surface Methodology (RSM). The chemical composition, morphology and structure of the alkaline catalyst were examined using Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Thermal properties of the catalyst were examined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The optimum parameters for obtaining a maximum amine density (3.65 mmol/g) in the catalyst were 79.8 DEA concentration, 80 °C reaction temperature, and 2.9 h reaction time. The catalyst was tested for transesterification of cottonseed oil with methanol. A maximum 88.6 conversion to fatty acid methyl ester (FAME) was achieved at optimum parameters of 33:1 methanol/oil molar ratio, 2.5 wt catalyst concentration and 60 °C reaction temperature. The results indicate that radiation induced grafting can be effectively used to prepare alkaline biocatalyst catalyst from flax fibres and the catalyst is a promising green candidate for biodiesel production. © 2019 Elsevier Ltd