This study synthesized Fe 3 O 4 nanoparticles of 30–40nm by a sonochemical method, and these particles were uniformly dispersed on the reduced graphene oxide sheets (Fe 3 O 4 /RGO). The superparamagnetic property of Fe 3 O 4 /RGO was evidenced from a saturated magnetization of 30emu/g tested by a sample-vibrating magnetometer. Based on the testing results, we proposed a mechanism of ultrasonic waves to explain the formation and dispersion of Fe 3 O 4 nanoparticles on RGO. A biosensor was fabricated by modifying a glassy carbon electrode with the combination of Fe 3 O 4 /RGO and hemoglobin. The biosensor showed an excellent electrocatalytic reduction toward H 2 O 2 at a wide, linear range from 4×10 −6 to 1×10 −3 M (R 2 =0.994) as examined by amperometry, and with a detection limit of 2×10 −6 M. The high performance of H 2 O 2 detection is attributed to the synergistic effect of the combination of Fe 3 O 4 nanoparticles and RGO, promoting the electron transfer between the peroxide and electrode surface.