The aim of this study was to evaluate the anaerobic co-digestion of diluted poultry manure (DPM) and used cooking oil (UCO). Mixtures of DPM with different UCO dosages (1 to 6% v/v) were prepared using a high-shear emulsifier and digested in batch anaerobic reactors. Increasing the UCO dosage increased the emulsion COD (from initially 64 to 182 g · L−1) but emulsion stability was affected adversely. UCO at the optimum dosage (1.5 to 2.0% v/v) was further digested in a semi-continuous mesophilic anaerobic reactor, to assess process feasibility at organic loading rates (OLRs) up to 5 g · L−1 · d−1. The reactor was stable, yielding biogas at 0.45 L · g−1 COD, with low supernatant COD (6 g · L−1), negligible VFA accumulation and no foaming.