Single-walled carbon nanotube (SWNT)/cellulose nanocomposite films were prepared using N-methyl-morpholine-N-oxide (NMMO) monohydrate as a dispersing agent for the acid-treated SWNTs (A-SWNTs) as well as a cellulose solvent. The A-SWNTs were dispersed in both NMMO monohydrate and the nanocomposite film (as confirmed by scanning electron microscopy) because of the strong hydrogen bonds of the A-SWNTs with NMMO and cellulose. The mechanical properties, thermal properties, and electric conductivity of the nanocomposite films were improved by adding a small amount of the A-SWNTs to the cellulose. For example, by adding 1 wt % of the A-SWNTs to the cellulose, tensile strain at break point, Young's modulus, and toughness increased similar to 5.4, similar to 2.2, and similar to 6 times, respectively, the degradation temperature increased to 9 degrees C as compared with those of the pure cellulose film, and the electric conductivities at phi. (the wt % of A-SWNTs in the composite) = 1 and 9 were 4.97 x 10(-4) and 3.74 x 10(-2) S/cm, respectively. Thus, the A-SWNT/cellulose nanocomposites are a promising material and can be used for many applications, such as toughened Lyocell fibers, transparent electrodes, and soforth. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 3588-3594, 2010