We demonstrate a nanosecond mode-locked erbium-doped fiber laser (EDFL) based on a reduced graphene oxide (RGO) saturable absorber (SA). The RGO SA is prepared by depositing the graphene oxide (GO) on fluorine mica through thermal reduction of GO. A scanning electron microscope (SEM), Raman spectrometer, and x-ray photoelectron spectroscopy (XPS) are adopted to analyze the RGO characteristics. The results show that the reduction degree of grapheme oxide is very high. By embedding the RGO SA into the EDFL cavity, a stable mode-locked fiber laser is achieved with a central wavelength of 1567.29 nm and repetition rate of 12.66 MHz. The maximum output power and the minimum pulse duration are measured to be 18.22 mW and 1.38 ns respectively. As far as we know, the maximum output power of 18.22 mW is higher than those of other nanosecond mode-locked oscillators reported. Such a nanosecond pulse duration and megahertz repetition rate make this mode-locked erbium-doped fiber laser a suitable seed oscillator for high-power applications and chirped pulse amplifications. © 2018 Chinese Physical Society and IOP Publishing Ltd.