Sm-Fe-N/Fe and Nd-Fe-B/Fe nanocomposite magnetic powders were successfully prepared by the use of sonochemistry to decompose the carbonyl iron compounds in decane solvents containing Sm-Fe-N and Nd-Fe-B powders at low temperature and in an inert atmosphere, respectively. The TEM observations and the corresponding EDS analysis show that the small nano-iron by sonochemical process are pure with a particle size of about 5 rim. The SEM observations indicate that the Sm-Fe-N and Nd-Fe-B powders are well coated by nano-sized Fe. The content coating Fe can be easily adjusted by controlling the carbonyl iron and the related sonochemical process. In addition, the preparation conditions and mechanism of coating process were discussed.