Design and preparation of highly efficient and stable cocatalysts are critical to the improvement of photocatalyst performance. A traditional cocatalyst consists of metal nanoparticles for the separation of photo-induced electron-hole pairs and for the reduction of protons. In this research we report a metal-semiconductor composite cocatalyst to increase light adsorption and to effectively enhance proton reduction capacity. A molybdenum rich molybdenum carbide (Mo-Mo2C) based noble-metal-free metal/semiconductor cocatalyst was loaded onto graphitic carbon nitride (g-C3N4) for highly efficient photocatalytic H-2 evolution from water. The Mo-Mo2C was synthesized via a temperature-programmed reaction using (NH4)(6)Mo7O24.4H(2)O as a precursor. The cocatalyst loaded 2.0 wt.% Mo-Mo2C/g-C3N4 composite photocatalyst has demonstrated excellent photocatalytic performance. The hydrogen evolution rate for the 2.0 wt.% Mo-Mo2C/g-C3N4 nanocomposites can be as high as 219.7 mu mol h(-1)g(-1), which is 440 times higher than that of g-C3N4 alone and 90% as high as 0.5 wt.% Pt/g-C3N4 photocatalyst (244.1 mu mol h(-1) g(-1)). Due to strong synergetic effects between Mo and Mo2C nanoparticles, this rate is 11.47 and 3.60 times higher than those for 2.0 wt.% Mo/g-C3N4 (19.1 mu mol h(-1) g(-1)) and 2.0 wt.% Mo2C/g-C3N4 (60.9 mu mol h(-1) g(-1)) photocatalysts respectively. Moreover, the 2.0 wt.% Mo-Mo2C/g-C3N4 catalyst is significantly stable for application in photocatalytic hydrogen evolution, with an apparent quantum efficiency of 8.3% one of the highest noble-metal-free efficiencies reported in literature. All results indicate that metal/semiconductor composites can serve as highly efficient cocatalysts for photocatalytic hydrogen evolution from water reduction.