POSS containing organometallic polymer (P2-Co) with dicobalt hexacarbonyl in the side chain was synthesized for solid-state pyrolysis to produce magnetic nanoparticles. For comparison, its analogue (P1-Co) without POSS groups was also prepared. After pyrolysis, the nanospheres obtained from P2-Co with POSS groups, had a Co@C-SiOx structure with small and uniform size (*15 nm), and its diameter almost remained unchanged when the pyrolysis temperature increased, showing excellent sinter-resistant properties. However, the nanoparticles obtained from P1-Co sintered seriously at high temperature, and its size was enlarged from 45 to 160 nm, when the pyrolysis temperature increased from 700 to 850°C. Meanwhile, the obtained nanoparticles all showed good magnetic properties, with the saturation magnetization in the range of 24.0-58.5 emu g -1 , making them promising candidates for the actual applications.