Objective:To study the effect of Knoevenagel condensation in room temperature ionic liquid. Methods:Knoevenagel condensation of aromatic aldehydes with active methylene compounds was proceeded smoothly in different room temperature ionic liquid. The ionic liquid could be recovered for reuse. Results:The yields of Knoevenagel condensation in room temperature ionic liquid was 85%~93%. The ionic liquid could be reused. When the reaction proceeded in room temperature ionic liquid with the same positive ion and different negative ions, the yields have little difference. Conclusion:The Knoevenagel condensation proceeds smoothly in room temperature ionic liquid under mild condition and with high yields. The ionic liquid can be reused, the process pollutes enrironment is little.