Morpholine was found to be an effective catalyst for the synthesis of 3-acetyl coumarin through Knoevenagel condensation of 2-hydroxybenzaldehyde with ethyl acetylacetate. Reaction conditions such as solvent, base, catalyst loading, and reaction time were carried out to scrutiny. The results showed that under ultrasonic condition, 2-hydroxybenzaldehyde reacted with ethyl acetylacetate in ethanol at room temperature catalyzed by morpholine and Na2CO3, giving 3-acetyl coumarin after 20 min in 90% yield. The product was characterized with NMR and IR. A plausible catalytic mechanism was also proposed.