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Metallic-phase MoS 2 exhibits Pt-comparable electrocatalytic hydrogen evolution reaction (HER) performance in acidic conditions. However, the controllable synthesis of metallic-phase MoS 2 is quite challenging because the key factor determining the phase types of MoS 2 during synthesis is still unclear. Herein, the effect of organic sulfur sources on the formed MoS 2 phase is studied by use of thioacetamide (TAA), L-cysteine, and thiourea as sulfur sources. The TAA and L-cysteine produce metallic MoS 2 , while thiourea gives rise to semiconducting MoS 2 . Owing to the metallic phase and smaller size, the MoS 2 prepared with TAA and L-cysteine has a higher electrocatalytic HER activity than the MoS 2 obtained from thiourea. The HER overpotential of MoS 2 synthesized with TAA is only 210 mV for reaching the current density of 10 mA/ cm 2 , and the corresponding Tafel slope is 44 mV/decade. Further studies find that the decomposition temperature of sulfur precursors is the key factor for the formation of metallic MoS 2 . Sulfur precursors with a lower decomposition temperature release sulfur ions quickly, which in turn stabilize the metallic phase and inhibit the growth of MoS 2 into large sizes. Our findings unveil the key factor for controlling the phase type of MoS 2 synthesized from organic sulfur precursors and will be very helpful for the synthesis of MoS 2 with high electrocatalytic activity.