Abstract: Identification of the variations of Cu and Zn during the anaerobic digestion of manures is important to evaluate their input and environmental safety after application of digested residues. In order to study the concentration variations and chemical form changes of Cu and Zn during the anaerobic digestion of pig and dairy manures, two bench-scale complete stirred tank reactors were carried out under mesophilic ((37±2)℃) condition. The results showed that:1) Cu and Zn concentrations in the digested pig slurry (DPS) distinctly increased with the anaerobic reaction time, whereas the variation trends in digested dairy slurry (DDS) were not obvious. 2) During a 158 d period of anaerobic digestion, the concentrations of Cu and Zn in DPS were significantly (P<0.001) lower than those in the raw pig slurry (RPS), with decrease rates of 80% and 43%, while those in DDS were slightly lower than those in raw dairy slurry (RDS), with decrease rates of 11% and 13%, respectively. 3) The percentages of dissolved Cu (filtered through 0.45 μm membrane) in DPS and DDS were 53% and 43% of the total Cu, respectively, while those of dissolved Zn were 35% and 19% respectively in DPS and DDS, with increase rates of 109% (P≤0.01) and 37%. 4) Cu and Zn in digested residues of pig manure and dairy manure were mainly oxidizable forms and residual and extractable/exchangeable forms, respectively. Mesophilic anaerobic digestion did not affect the removal of dissolved Cu and Zn in pig and dairy manures. In addition, the bioavailability of Cu and Zn would be enhanced due to the transformation of residual fractions of Cu and Zn in pig and dairy manures during the anaerobic digestion. This is not favorable for land application.