Magnetic nanoparticles (MNPs) are used as enzyme mimics owing to the unique properties such as good catalytic activity, high stability and rapid separation. Here, a range of fatty acid (C₆–C₁₁) coated Fe₃O₄ nanoparticles were synthesized, and they possess excellent peroxidase-like catalytic activity, which can catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to produce a blue coloration in the presence of hydrogen peroxide (H₂O₂). The fatty acid coated Fe₃O₄ nanoparticles can remarkably enhance peroxidase-like activity compared to Fe₃O₄. The study showed that the appropriate chain length can effectively improve the catalytic efficiency and heptylic acid coated MNPs (Fe₃O₄@C₇ MNPs) showed better peroxidase-like enzyme activity. Based on the steady-state kinetic analysis and Michaelis–Menten model, Fe₃O₄@C₇ MNPs are similar to that of horseradish peroxidase (HRP), indicating their superior affinity for the substrates TMB and H₂O₂. The peroxidase-like Fe₃O₄@C₇ MNPs catalytic activity was successfully applied for the detection of uric acid (UA) with a limit of detection (LOD) of 2.8 μM. The linear regression equation for uric acid substrate was stated as y = 0.001x + 0.163 with a correlation coefficient of 0.992. The method has the advantages of high sensitivity, good selectivity and cost-effectiveness, and provides an efficient and simple method for the determination of UA concentration in human serum.