The crude methylna phthalene oil (CM NO) contains some nitrogen impurities, such as quinoline , isoquinoline and indole and soon. The nitrogen compounds can be removed off efficiently from CM NO by extraction with formic acid.Experiment was carried out to investigate the effect of the extraction time , temperature , formic acid concentration , volume ratio of solvent to oil on extraction. The result shows that extraction system reaches equilibrium within 3 min for quinoline , isoquino line and methylquinoline , but needs longer time for indole , the formic acid concentration has little effect on removal of quinoline , isoquinoline and methylquinoline but has strong effect on removal of indole , the higher the formic acid concentration,the higher the indole removal efficiency , the removal efficiency for indole is higher below 40 ℃. The yield for me thylnaphthalene is more than 95%, which presents an effective method for removal of nitrogen compounds from CM NO.