Recycling of waste cooking oil greatly reduces the environmental impact of its disposal. As fats can give rise to high methane yields, the use of waste cooking oil for biogas production seems a promising solution. The aim of this work was to test the anaerobic digestion performances of waste cooking oil in co-digestion with pig slurry, the fat degradation dynamics and the relationships between digestion performances and pig slurry compositional properties. In laboratory batch, static, mesophilic anaerobic conditions, the treatments waste cooking oil in synthetic hydration medium (WCO + HM) and waste cooking oil in pig slurry (WCO + PS) were compared. Pig slurry alone was included for reference. Co-digestion with pig slurry greatly shortened the lag phase (by 58.3%) and increased the overall methane production per reactor (by 15.5%). An increase in emulsion stability of the biphasic system and an earlier triglyceride degradation were associated with the better anaerobic digestion performances in the WCO + PS reactors. A negative, however reversible, effect of palmitic acid accumulation on methane production was also observed. These results are encouraging for the application of co-digestion of waste cooking oil and pig slurry in agricultural biogas plants.