We demonstrated an erbium-doped mode-locked fiber laser using a single-walled carbon nanotube- dispersed polyimide (SWNT-PI) film. Different mode-locking operations were compared and analyzed utilizing SWNT-PI films with different concentrations (2, 1, and $0.25\text{\hspace{0.17em} wt.}\%$ , respectively). It was found that the continuous single-pulse mode-locking operation was often accompanied by a continuous wave oscillation part for the 1 and $0.25\text{\hspace{0.17em} wt.}\%$ SWNT-PI films, whereas the $2\text{\hspace{0.17em} wt.}\%$ SWNT-PI film presented the most excellent mode-locking performance, thanks to sufficient modulation depth. Using the $2\text{\hspace{0.17em} wt.}\%$ SWNT-PI film, a stable pulse train with a pulse width of $840\mathrm{fs}$ and a repetition rate of $15.3\mathrm{MHz}$ was achieved. The average output power was $0.33\mathrm{mW}$ at the pump power of $155\mathrm{mW}$ under an output coupling ratio of 10%. Operational performance of the laser cavity when employing the $2\text{\hspace{0.17em} wt.}\%$ SWNT-PI film was also demonstrated.