Abstract CTAB coated CoFe2O4 samples were synthesized by hydrothermal method at 60 °C (CT-60) and 120 °C (CT-120) respectively. X-ray diffraction (XRD), Transmission electron microscopy (TEM) and magnetic measurements exhibits the strong corelation between structural and magnetic properties. The presence of CTAB in as-prepared samples was confirmed by Fourier-transform infrared spectroscopy (FTIR) and chemical shift of proton nuclear magnetic resonance (1H-NMR) spectra. The peak to peak linewidth Δ H P P = 34 mT and Δ H P P = 78 mT at 300 K was estimated for CT-60 and CT-120 respectively using electron paramagnetic resonance (EPR) measurements. The resonance linewidth and the relaxation phenomenon of EPR signal is affected by the particle size. The estimation of relaxation time and relaxivity was done using resonance linewidth of EPR signal. The spin-spin relaxation time T 2 is found to be decreases as particle size increases and spin-lattice relaxation time T 1 increases with particle size. The higher relaxation efficacy is estimated for CT-120 than CT-60 sample. The enhancement in saturation magnetization M S and magnetic anisotropic constant K e f f in CT-120 than CT-60 is due to stronger interparticle interactions in CT-120 compared to CT-60.