DC conductivity of conjugated polymer-single-walled carbon nanotube (SWNT) composite films has been measured for different SWNT concentrations, The composite was prepared by dispersing SWNTs in the poly (3-octylthiophene), P3OT matrix already dissolved in xylene. The conductivity of the composite films showed a rapid increase as the SWNT concentration increases beyond a certain value. This behavior is explained in terms of percolating paths provided by the SWNTs in the volume of polymer matrix. To investigate the effect of length of nanotubes on the percolation conductivity, different SWNT samples were employed with similar diameter but varying tube lengths. It was found that the conductivity of the composite films is strongly dominated by the length of the nanotubes. Lower percolation limit and high conductivity value of composite films is observed for longer nanotubes. Furthermore, the conductivity is observed to be dependent on the size of the host polymer molecule also. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 89-95, 2010