Ultrasmall molybdenum sulfide nanoparticles with diameters of 1.47 0.16 nm were fabricated from bulk MoS2 by a combination of ultrasonication and centrifugation. The nanoparticles were then assembled on an Au surface to form a film with high electrocatalytic activity for hydrogen evolution reaction (HER). A Tafel slope of 69 mV per decade was measured for this film and the onset potential was estimated to be 0.09 V. The small loading (1.03 mg/ cm2) and the high current density (0.92 mA/ cm2 at n=0.15 V)demonstrated extremely high catalytic efficiency. X-ray photoelectron spectroscopic results revealed that the assembled nanoparticle film was sulfur enriched with abundant S edges and a structural rearrangement of the S rich particles might occur during the self-assembly process, resulting in significantly enhanced electrocatalytic activity for HER. Electrochemical impedance measurements suggested that the assembling process optimized the conductivity of the nanoparticle film, which contributed to the enhanced HER catalytic activity. Our research has provided a new way to synthesize active molybdenum sulfide nanoparticles for HER and a new approach to achieve enrichment of S edges on molybdenum sulfide, which might have potential use not only for electrocatalytic HER, butalso for photoelectrocatalytic HER and plasmon-enhanced water splitting.