Polyoxymethylene was reinforced with nano-sized inorg. vinyl group-contg. polyhedral oligomeric silsesquioxanes (winPOSS) 0.5 and 1.0% by mass in molten state by extrusion in a single screw extruder. Mech. properties and structural changes of the nanocomposite materials were detd. by dynamic mech. thermal anal. and scanning electron microscopy, equipped with an energy-dispersive X-ray analyzer. The addn. of winPOSS nanoparticles resulted in increase in mech. strength (higher stiffness) and some structural changes of the nanocomposites. The nanocomposites showed high homogeneity.