Nitrogen Inhibition during an anaerobic co-digestion process was studied in this work.The substrate and inoculum used were obtained from a thermophilic biogas plant Sobacken,situated in Borås, Sweden. The batch experiments have been carried out in triplicate reactorswith different concentrations of ammonia ranging from 2400mg/l to 3400mg/l. The batchexperiment was working well for the all the concentrations of ammonia investigated. Theaverage methane yield was around 0.65 Nm3 CH4/kgVS for all the reactors. The laboratorywork has been further proceeded with a continuous process having two reactors working inparallel. Reactor 1 containing only substrate and the Reactor 2 contain substrate with surplusammonia added to make final concentration of 3400mg/l. The reactors were operated atorganic loading rate (OLR) of 3.3gVS/l/day and hydraulic retention time (HRT) of 20 days.Both reactors worked well for 29 days. During a period of an initial stable operation, theaverage methane production of Reactor 1 was 0.59 Nm3CH4/kgVS/day and for Reactor 2 theproduction rate was 0.56 Nm3CH4/kgVS/day. Then Reactor 1 showed a steady decrease in pHand methane production, while Reactor 2 showed stable operation for a few days longer withdecreasing pH and methane production only from day 36. The composition of substrate wasnot optimal; therefore the inhibition level of ammonium could not be determined.