Performance of anaerobic digestion (AD) of Palm Oil Mill Effluent (POME) with nitrogen-rich organic waste such as Waste Activated Sludge (WAS) was investigated in the batch experiment and continuous operation. In batch experiments, it was shown that the AD of POME needs sufficient alkalinity to maintain the stable pH. The alkalinity can be provided from inorganic chemical such as sodium bicarbonate as well as nitrogen-rich organic waste such as WAS. WAS was preferable than sodium bicarbonate for alkalinity source due to the synergistic and complementary effect of WAS with POME. The high content of total ammonia nitrogen in WAS made it possible to maintain stable pH and to enhance the performance of AD of POME. The methane yield in the mixture of POME and WAS were 22% and 81% higher than POME or WAS alone respectively. The methane production rate was 2 folds of WAS and 0.7 folds of POME. It also found that there was almost no lag period in the AD of POME and WAS. Thus, the mixing ratio of POME and WAS 40:60 which was in equivalent to COD/N ratio of 22 was necessary for the optimum methane production on the AD of POME and WAS. In the continuous operation, it was also shown that AD of POME and WAS with COD/N ratio of 22 was successfully operated with the methane yield improved up to 117% in the OLR of 1.5 g COD/L/d compared to WAS alone.