The experiment was carried out to investigate the effect of adding palm oil sludge in cattle diets on methane and volatile fatty acid production as well as on the microbial population of the manure when digested anaerobically. Four dietary treatments (T1, T2, T3 and T4) were formulated by the addition of palm oil sludge to a grass x legume silage at 0, 10, 20 and 30%, respectively. The percentage of methane on the total gas volume was measured with Sperian gas analyser while volatile fatty acids were measured using Spectrophotometer. Microbial population, digester pH and ambient temperature were also measured using microbial analysis method by Harrigan and McCance 1990, pH meter and thermometer, respectively. The experiment lasted for 90 days. Results showed that there were significant differences (p<0.05) in total gas production, methane production, acetic acid, propionic acid, butyric acid and microbial population among the different treatments. From the foregoing, it was concluded that palm oil sludge inclusion in cattle diet at 30% reduced methane production and acetic acid production while butyric acid and propionic acid production increased in cattle manure.