Monodispersed MoS2ultrathin nanosheets have been successfully fabricated by a facile hydrothermal process assisted by ionic liquid Brij56. The effect of Brij56 on the morphology and structure of MoS2has been obviously observed. XRD shows that the as-prepared MoS2assisted by Brij56 has the weak and broad peak of (002) planes, which implies the small size and well dispersed structure of MoS2nanosheets. TEM and SEM images reveal that MoS2ultrathin nanosheets have small size and few stacking layers with the adding of Brij56. HRTEM images prove that MoS2appears to have a highly monodispersed morphology and to be monolayer ultrathin nanosheets with the length about 5–8 nm, which can provide more exposed rims and edges as active sites for hydrogen evolution reaction. Brij56 has played a crucial role in preparing monodispersed MoS2ultrathin nanosheets as excellent electrocatalysts. The growth mechanism of monodispersed MoS2has been discussed in detail.