Large eddy simulation (LES) of a passive flow control application, namely Jet Boat Tail (JBT), on a base configuration is performed. LES results indicate that a significant reduction of base drag by 15.6% is achieved using the JBT passive flow control. Two different flow structures are detected for the baseline model. The lower outstanding frequency of velocity fluctuation is attributed to the large scale instability of vortex shedding, whereas the higher frequency is the result of small scale instabilities from the separation of shear layer. For the baseline model, large scale vortex structures are dominant. For the JBT model, The periodically pulsing jet interacts with the base flow and free stream shear layer. The interactions generate vortex pairing and enhance the mixing and entrainment effect. Higher frequencies flow entrainment is dominant for the JBT model.