The diesel fuel hydrocarbon prevents the growth of a lot of microorganisms in the wastewater and causes massive loss of aquatic life. Removal of diesel fuel from water can be very difficult and have environmental impacts. In this study, four different biochar-based magnetic adsorbents biochar/Fe3O4 (BC/Fe3O4), ethylamine functionalized-biochar/Fe3O4 (E-BC/Fe3O4), butylamine functionalized-biochar/Fe3O4 (B-BC/Fe3O4) and octylamine functionalized-biochar/Fe3O4 (O-BC/Fe3O4) were synthesized and characterized for diesel fuel adsorption from water. The Response Surface Methodology-Box-Behnken design was employed to optimize adsorption key factors like dosage of adsorbent, mixture pH and contact time. Addition of alkyl amine groups made the biochar resistant to adsorption of water and enhanced the hydrophobicity of biochar. Modification of the biochar surface by alkyl amine groups can improve the adsorption properties of the biochar with increasing the surface area and hydrophobicity of the adsorbents, which led to an enhancement in the adsorption capacity. An increase in carbon chain length led to an increase in the adsorption efficiency of diesel fuel. The adsorption capacity for the O-BC/Fe3O4 adsorbent was very high value and it was six times higher than that of the BC/Fe3O4 adsorbent. Also, magnetization by nanoparticles of Fe3O4 deposition onto biochar allows the recovery of the adsorbents via an external magnetic field. Kinetically, the experimental results of adsorption well fitted the second-order model, also results revealed that all adsorbents followed Friendlich isotherm model. The maximum capacities for O-BC/Fe3O4, B-BC/Fe3O4, E-BC/Fe3O4 adsorbents were 1921.8, 6750, 11250 and 14531.3 mg/g, respectively. Moreover, thermodynamic study indicated that adsorption on the all adsorbent was spontaneous and the values of enthalpy as well as entropy were positive.