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Carbon nanotube (CNT) films made from sing-wall and multi-wall nanotubes (SWCNTs and MWCNTs) were investigated recently as strain gauge for strain sensing. The quality of CNT films is important thereby as well as for many related applications. In this paper relevant processing factors such as the sonication time and concentration of sodium dodecyl sulfate (SDS) are investigated to characterize their influences on electrical properties of the produced CNT films. The results show that sonication time plays an important role for the quality of CNT films. A longer sonication time realizes better homogeneity of the CNT films, lower resistance values and higher reproducibility of the specimens. SWCNT and MWCNT specimens were manufactured and measured for strain sensing. MWCNT specimens show a better sensing property than SWCNT specimens with an applied strain up to 1%. Two strain sensing regions for MWCNT films can be clearly recognized possibly due to the different response characteristics of contact resistance and tube resistance to external loading. As the strain is smaller than 0.1% the sensitivity of the MWCNT films is low, about 2.5. For the strain from 0.1% to 1% the sensitivity is about 5 and the sensing property is reproducibility. Results of SWCNT films are much complex compared to MWCNT films. For the strain smaller than 0.2% the resistance variation curves and the sensitivities of SWCNT films are similar to those of MWCNT films. The sensitivities decrease however for the strain from 0.2% to 0.3% and change dramatically and irreproducibility for the strain from 0.5% to 1%.