Objective: To study the role of serum Monoamine oxidase (MAO) in Lichen planus(LP). Methods: 82 cases of Lichen planus patients and 35 healthy controls were selected in the investigation. The total serum MAO levels were measured. Results: The levels of serum MAO in Lichen planus patients were significantly higher than those in healthy controls (P<0.01). The severity of Lichen planus was not correlated with serum MAO levels (r =0.4873, t=0.73, p>0.05). Conclusion: According to the findings, there might be a coincidence of MAO and lichen planus. However, further studies are required to clarify the immunological mechanisms which are responsible for MAO synthesis during immunoreaction.