This work studied the effects on the anaerobic digestion of sewage sludge by zero valent iron nanoparticles (NZVI) dosage. Biochemical methane potential tests were carried out with 5-9 mg/g(VS) (99.7%, 40-60 nm). The biogas yield increased from 132 (control) to 310 mL/g(VS) with 9 mg/g(VS). The methane content increased from 63.2% (control) to 77.6% with NZVI, which corresponded to a maximum yield of 238 mLCH(4)/g(VS) with 9 mg/g(VS). The maximum VS reduction was 19.6%. The highest INT-ETS activity (20.1-37.1 mu gINT(red)/g(VS).h) corresponding to the maximum values of sCOD was reached within the first days. NZVI decreased the ORP to -300 mV and increased the VFA's concentration (+ 2000 mg/L). The ORP-VFA-pH analysis showed that NZVI promoted the acidogenesis-acetogenesis without acidification. That is, NZVI was effective in intensifying the performance and stability of the process.