Dispersion behavior of multiwalled carbon nanotube (MWCNT), rheological and mechanical properties of various MWCNT/poly(styrene-co-acrylonitrile) (SAN) nanocomposites were investigated. MWCNT/SAN nanocomposites were prepared by three different methods; MWCNT/SAN melt blending, MWCNT/SAN in situ atom transfer radical polymerization (ATRP) and functionalized-MWCNT/SAN in situ ATRP. Formation of SAN onto the surface of MWCNT and the molecular weight of grafted-SAN were confirmed by fourier transform infrared spectra, H-1-NMR and C-13-NMR. Crossover frequency of storage and loss modulus from rheological measurement and dynamic mechanical analysis showed that functionalized MWCNT/SAN in situ ATRP nanocomposite showed more uniform dispersion of MWCNT. Improved mechanical and electrical properties were observed for functionalized MWCNT/SAN in situ ATRP nanocomposite. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 2930-2936, 2010