The present invention discloses a nano-granule fuel oil, which substantially contains no granule greater than 10 nm. This nano-granule fuel oil can be obtained by treating conventional fuel oils under a magnetic field with a air gap magnetic field intensity of at least 8000 Gauss and a magnetic field gradient of at least 1.5 tesla/cm. The nano-granule fuel oil of the present invention significantly improves the combustion degree of fuel oil as compared with the conventional fuel oils. Its application can considerably economize fuel oil and reduce the emissions of CO and the like in tail gas.