An emerging contaminant, micron-sized zero valent iron (mZVI) has been reported to be accumulated in waste activated sludge (WAS). In the present study, the potential effects of mZVI on WAS anaerobic fermentation performance, as well as the shift of the microbial community composition and function, were assessed. Results from batch experiments indicated that a particular concentration of mZVI (1.5 g/L, in the range of 0.0 to 5.0 g/L) improved volatile fatty acids (VFA) accumulation by 4.84 times that in the control group, the ability (dosage dependent) to remove phosphorus, and the dewaterability of fermented WAS from 126 +/- 5 s (control group) to 104 +/- 3 s (group with 1.5 g/L mZVI). Furthermore, high-throughput sequencing revealed that mZVI had no significant impact on the shift of microbial community structure, but directly stimulated the functional performance related to anaerobic fermentation. This study will provide new insights into the mZVI application and its effect on WAS utilization.