A simple protocol was established to synthesize various coumarins using Tetramethylammonium hydroxide as a phase transfer catalyst in water. For coumarin synthesis, the active methylene compounds such as diethyl melonate, meleno nitrile, ethyl aceto acetate, was used with salicylaldehyde, and 2-hydroxy-1-naphthaldehyde in the presence of catalytic amount of piperidine in water and Tetramethylammonium hydroxide. This process is a practical synthetic method for the preparation of various 3-substituted coumarins and benzocoumarins. The influence of surfactant inwater on the Knoevenagel is demonstrated. The products formed were 100% pure and high yield and coumarin product could simply be separated via filtration.