Poly(glycidyl methacrylate) grafted Linum usitatissimum (flax) fibers functionalized with diethylamine (DEA) groups followed by alkalisation were prepared and used as a heterogeneous catalyst for production of biodiesel. Particularly, the new basic catalyst was used for transesterification of cottonseed oil using different molar ratios with methanol and various reaction temperatures. The gas chromatography analysis was used to confirm the conversion of the cottonseed oil to biodiesel. The transesterification reaction temperature affected the conversion percentage significantly. The highest conversion was obtained at 60 °C. In addition, the oil/methanol ratio in the reaction mixture of 1:33 resulted in the highest conversion ratio reaching about 97 %. These results suggest that the alkaline organic catalyst prepared in this study has a potential for application in biodiesel production.