Controlling free ammonia in an anaerobic fermenter at pertinent levels is reported recently to be an economically attractive and practically feasible approach to enhance hydrogen yield from waste activated sludge (WAS). This paper reports a new technology for WAS dark fermentation, i.e., using heat pretreatment (70 °C for 60 min) to assist free ammonia for further improving hydrogen yield. The experimental results showed that the accumulative hydrogen production from combined reactors was promoted from 12.3 to 19.2 mL/g VSS (volatile suspended solids), the maximum of which was 1.8, 2.7, and 7.1 times of that from sole free ammonia (131.9 mg NH 3 -N/L), sole heat, and blank reactors, respectively. Mechanism explorations showed that the combination strategy significantly enhanced WAS disintegration, providing more substrates for hydrogen production. Moreover, the combination suppressed activities of all microbes associated with anaerobic fermentation, but its inhibition to hydrogen consumers was much severer than that to other microbes.