Solvent extraction of nitrogen compounds from catalytically cracked diesel oil was studied by using a volatile carboxylic acid, such as acetic acid. The complexes of nitrogen compounds with acetic acid could be thermally decomposed, yielding nitrogen compounds and the volatile acid for recycle. Catalytically cracked diesel oil from Shengli Petrochemical Corporation was used as feed oil. Nitrogen compounds, especially basic nitrogen compounds, were effectively separated with aqueous acetic acid solution by solvent extraction based on chemical complexation. The stability of catalytically cracked diesel oil was greatly improved. Experimental conditions for the separation of nitrogen compounds were investigated. The recovery of catalytically cracked diesel oil was quite high, and chemicals were not consumed, avoiding pollution of environment. The new method sees a promising potential in industry.