This study aimed to assess the system stability and synergistic effects of co-digesting pig manure (PM) and grass silage (GS) in a pilot-scale study. Anaerobic digestion of PM alone and co-digestion of PM with GS was carried out in a 480-L continuously stirred tank reactor. The experiment consisted of two phases. In Phase I, PM was digested at an organic loading rate (OLR) of 0.87 kg volatile solid (VS) m−3·d−1, and in Phase II, PM and GS were co-digested at 1:1 on a VS basis at an OLR of 1.74 kg VS·m−3·d−1. The pilot-scale anaerobic digestion system was stable in both phases. At the steady state, average pH and free ammonia concentrations were 7.99 and 233.0 mg l−1 in Phase I and were 7.77 and 158.3 mg l−1 in Phase II, respectively. The specific methane yields increased from 154 ml CH4/g VS added in Phase I to 251 ml CH4/g VS added in Phase II. On average, soluble chemical oxygen demand and VS removal efficiencies increased from 81.4% and 41.4% in Phase I to 87.8% and 53.9% in Phase II, respectively. Further evaluation of synergism suggests that co-digestion of PM and GS can improve system stability and biogas yields despite marginal synergistic effects at pilot-scale.