Nylon 12 blends (BP) with nitrile-butadiene ultrafine full-vulcanized powdered rubber (NBUFPR) were successfully synthesized in a corotating intermeshing twin screw extruder via anionic ring-opening polymerization of lauryllactam at 250 degrees C. The in situ formed blends were characterized by mechanical test, fourier transform infrared spectroscopy, scanning electron microscopy, and thermogravimetric analysis. A brittle-tough transition occurs when NBUFPR content is 4 wt%. And the notched Izod impact strength achieves 989 J m(-1) when 5 wt% NBUFPR was added. Because of the good dispersion property of NBUFPR during the process of reactive extrusion, the interparticle distance decrease remarkably. Besides, the in situ formed NBUFPR-graft-PA12 leads to better adhesion between the two phases. For comparison, another series of blends (BL) were also prepared by melt blending hydrolytic-polymeric PA12 and NBUFPR in the same twin screw extruder. Brittle-tough transition did not appear in BL blends within the same range of NBUFPR content. POLYM. ENG. SCI., 50:1178-1185, 2010. (C) 2010 Society of Plastics Engineers